- Lee Philips - Radio DJ Gene 'Buddy' Maxwell
- Jill Haworth - Cathy Evans
- Alan Baxter - Col. Grover
- Henry Darrow - Policeman
- Miriam Hopkins - Mary Kry
- Olive Deering - Lisa Lawrence
- Carroll O'Connor - Deimos
- BarBara Luna - Gaby Christian
- Diana Sands - Dr. Julia Harrison
- Nick Adams - Mike Benson
- Salome Jens - Laurie Cameron
- Patrick O'Neal - Jonathan Meridith
- Eddie Albert - Andy Thorne
- Dewey Martin - Jim Darcy
- Burt Metcalfe - Eddie Phillips
- Bert Remsen - Frank Summers
- Billy Green Bush - Tom - Duckhunter #3 (as William Bush)
- Donald Pleasence - Harold J. Finley
- Edward Mulhare - Professor Mathers
- Jeff Corey - Byron Lomax
- Ron Randell - Henderson James
- Shirley Knight - Noelle Anderson
- Alfred Ryder - Edgar Price
- Ron Hayes - Pilot Coberly
- Ed Asner - Detective Sgt. Thomas Siroleo
- Michael Tolan - Prof. Stephen Grave
- Grace Lee Whitney - Carla Duveen
- Larry Pennell - Dr. Evan Marshall
- Geoffrey Horne - Wade Norton
- Nancy Malone - Laura Hanley
- John Anderson - Ebonite Interrogator
- Howard Da Silva - Thurman Cutler (as Howard da Silva)
- William Shatner - Gen. Jeff Barton
- June Havoc - Karen Thorne
- Mary Murphy - Linda Darcy
- Leslie Stevens - Election Returns Commentator (voice) (uncredited)
- Leonard Stone - Dr. Phillip Gainer
- John Marley - Dr. Sigmund Hindeman
- Gary Merrill - Dr. James Hamilton / Major Roger Brothers
- Francis De Sales - Prison Warden (as Francis de Sales)
- Ralph Meeker - John Dexter
- Russell Collins - Justice of the Peace
- Ruth Roman - Prof. Diana Brice
- Lloyd Nolan - Tom Kagan
- Sam Wanamaker - Dr. Simon Holm
- Michael Forest - Prof. Stuart Peters
- Bruce Dern - Ben Garth
- Barry Morse - Phobos One
- Vera Miles - Kassia Paine
- Martin Sheen - Pvt. Arthur Dix
- Peter Lind Hayes - Dr. James Stone
- Michael Constantine - Joe Dix
- Kay E. Kuter - Limbo Being
- Peggy Ann Garner - Amanda Frank
- Mark Roberts - Bob Conner
- Paul Lambert - Dr. Henschell
- Nora Marlowe - Mrs. Ives
- Peter Breck - Sen. Orville
- Harry Guardino - Major Roger Brothers / Dr. James Hamilton
- Jerry Douglas - Tom Evans
- Melinda Casey - Vivia Balfour Hayden
- Robert Webber - Ikar
- Phyllis Love - Andrea Holm
- Gladys Cooper - Mrs. Palmer
- Michael Higgins - Dr. Thomas Kellander
- Ed Nelson - Col. Luke Stone
- William Boyett - Co-Pilot Beeman
- Cliff Robertson - Alan Maxwell
- Phillip Pine - Ted Pearson
- Frank Maxwell - Dr. Keenan
- Harry Townes - Dr. Clifford Scott
- Martín Garralaga - Paco
- Buck Taylor - Gard Hayden
- Nina Foch - Eva Fraser
- Cedric Hardwicke - Colas (as Sir Cedric Hardwicke)
- Adam West - Major Merritt
- Philip Abbott - Ben Fields
- Philip Abbott - Lincoln Russell
- David Frankham - Harvey Kry Jr.
- David Frankham - Capt. Terrence Ralph Brookman
- Ford Rainey - D.A. Thomas Coyle
- Arthur Hunnicutt - Lamont
- Sidney Blackmer - William Lyons Selby
- Priscilla Morrill - Vera Finley
- Joe De Santis - Colonel William Campbell
- Henry Silva - Chino Rivera
- Henry Silva - General Juan Mercurio
- Janet Blair - Lynn Arthur
- Kent Smith - Dr. Block
- Kent Smith - Aabel
- Barry Jones - Dwight Hartley
- Bernard Kates - Dr. Whorf
- Skip Homeier - Dr. Roy Clinton
- Grant Williams - Major Douglas McKinnon
- Janos Prohaska - Thetan (uncredited)
- Janos Prohaska - The Mikie
- Richard Loo - Li-Chin Sung
- William Stevens - Navigator Dexter
- William Stevens - Policeman
- James McCallion - Dr. Tremaine
- Jay Novello - Prof. Arivello
- Gene Raymond - Sawyer
- Whit Bissell - Commanding General
- James Hong - Wen Li
- Richard Tretter - Radio Engineer
- James Yagi - Li Kwan
- Joseph V. Perry - Police Chief (as Joseph Perry)
- Don C. Harvey - Military Commander (as Don Harvey)
- Mavis Neal Palmer - Woman (as Mavis Neal)
- Clarence Lung - Maj. Ho Chi-Wong
- William Bramley - "Baldy" Baldwin
- Dorothy Green - Matron
- Coby Denton - Sentry
- Paul Kent - Detective
- Anne Loos - Emily Radcliffe
- Harry Ellerbe - Finley's Doctor
- Fred Beir - Steve Crandon
- Constance Cavendish - Gert 'the Bread' Evans
- George Pelling - Policeman
- Martin Wolfson - Dr. Herschel
- Clay Tanner - Fred - Duckhunter #1
- Hal Bokar - Bert - Duckhunter #2
- Martin Landau - Andro
- Martin Landau - Richard Bellero
- Lee Zimmer - Carl Ford - TV Announcer
- Geraldine Brooks - Yvette Leighton
- Geraldine Brooks - Ann Barton
- John Considine - Lt, Bertram Cabot
- Joan Camden - Prof. Stephanie Linden
- Joan Camden - Ann Pearson
- Maxine Stuart - Mrs. McCluskey
- Karl Held - Capt. Joseph Reardon
- Jack Raine - Old Man (scenes deleted)
- Joanne Gilbert - Barbara Scott
- C. Lindsay Workman - Dr. Anderson (as Lindsay Workman)
- Robert Benevides - Capt. James Harrison (as Robert Beneveds)
- Aki Aleong - Akada
- Aki Aleong - Dr. Sui-Lin
- Don Gordon - Luis B. Spain
- Don Gordon - Dave Crowell
- John Newton - Peterson
- Matty Jordan - Sentry
- Barry Atwater - Dr. Jonas Temple
- Ken Renard - Caretaker
- William Douglas - Henry Castle
- William Douglas - Galaxy Being (as Wm. O. Douglas)
- David Garner - Ralph
- David McCallum - Gwyllm Griffiths
- David McCallum - Tone Hobart
- Bill Gunn - Lt. James P. Willowmore
- Sally Kellerman - Ingrid Larkin
- Sally Kellerman - Judith Bellero
- Lillian Adams - Dix's Mother
- Lisa Mann - Krug's Governess
- Allyson Ames - Loreen
- Allyson Ames - Arndis Pollard
- Gene Darfler - Warren Edgar Morley
- Tom Palmer - Coroner
- Noel De Souza - Capt. Fortunado
- Edward Colmans - Servant
- Scott Marlowe - André
- Scott Marlowe - Jory Peters
- Jon Silo - Oswaldo
- Stuart Lancaster - Skipper
- Robert F. Simon - Gen. Maximillian R. Hart
- Ivan Dixon - Sgt. James Conover
- Ivan Dixon - Major Harold Giles
- Claude Woolman - Maj. Roger Hill
- Ron Foster - Dr. Robert Richardson (as Ronald Foster)
- Gene Tyburn - Goldsmith
- Don Ross - Haddon
- Anthony Jochim - Dr. Mordecai Spazman
- John Hoyt - Emmett Balfour
- John Hoyt - Bifrost Alien
- John Hoyt - Professor Hebbel
- Dee Hartford - Mrs. Clarke
- Marsha Hunt - Francesca Fields
- Robert Culp - Trent
- Robert Culp - Paul Cameron
- Robert Culp - Allen Leighton
- Joanna Frank - Regina
- Robert Duvall - Adam Ballard
- Robert Duvall - Louis Mace
- Booth Colman - Doctor Howard Warren
- Tony Mordente - Genero Planetta
- Chris Warfield - Johnny
- Richard Dawson - Oliver Fair
- Len Lesser - Sforza Water/Power worker
- John Graham - Supervisor
- Willard Sage - First Reporter
- Willard Sage - Coulter
- Willard Sage - Chief of Staff
- Jason Wingreen - Fred Severn
- Jason Wingreen - Bill Turner
- Jason Wingreen - Coroner Leland
- Chita Rivera - Mrs. Dame
- Neil Hamilton - Richard Bellero, Sr.
- Neil Hamilton - Hilary J. Clarke
- Edward Platt - Mr. Terrence
- Edward Platt - David Hunt (as Edward C. Platt)
- Edward Platt - Dean Radcliffe
- John Milford - John Bartlett
- Nellie Burt - Justice's Wife
- Nellie Burt - Ethel Latimer
- Stephen McNally - Col. MacWilliams
- Walter Burke - Recruiter
- Walter Burke - Dr. Frederick Riner
- Richard Jaeckel - Capt. Mike Doweling
- Burt Douglas - Man
- Peter Mark Richman - Jefferson Rome
- Peter Mark Richman - Prof. Ian Fraser
- Simon Oakland - Empyrian
- Robert Osterloh - General
- James Shigeta - AIO Captain Newa
- James Shigeta - Maj. Jong
- John Kellogg - Major Nathan Jennings
- Jacqueline Scott - Carol Maxwell
- Jacqueline Scott - Dr. Alicia Hendrix
- Robert Doyle - Marc Lake
- Robert Doyle - Wilt Morgan
- Joe E. Tata - Deputy (as Joey Tata)
- Sam Reese - George Wilkerson (as Sammy Reese)
- Sam Reese - Clyde Wyatt
- Peter Baldwin - Lt. Gordon Halper
- Douglas Henderson - Dr. Tillyard
- Douglas Henderson - Detective Lt. Runyan
- Douglas Henderson - Dr. Paul Fredericks (uncredited)
- George Macready - Larry K. Hillerman
- George Macready - Dr. Marshall
- Russell Johnson - Major Benedict
- Yale Summers - Buddy Lyman
- Arthur Batanides - Lt. Ken Galvin
- Arnold Merritt - Tommy Shadbury
- Mimsy Farmer - Denise Ward
- John McLiam - Arjay Beasley
- Shirley O'Hara - Receptionist
- Shirley O'Hara - Dr. Soldini
- Alex Nicol - Gen. Lee Stocker
- Warren Oates - Reese Fowler
- Angela Clarke - Sueann Beasley
- Ben Wright - Gen. Benton
- Ben Wright - Elder of Luminos
- Ben Wright - Philip Exeter Dundee
- Robert Sampson - Lt. Peter Chandler
- Peter Madsen - Carter - Watchman
- Peter Madsen - Reporter #2
- Hari Rhodes - Lt. Ernie Travers
- Herman Rudin - Prof. Henry Lacosta
- Richard Derr - Phillip Griffith
- Luana Anders - Theresa Ames
- Macdonald Carey - Roy Benjamin
- Betsy Jones-Moreland - Julie Griffith
- Gloria Grahame - Florida Patton
- Richard Ney - Mr. Zeno
- Bill Hart - Calco Galaxy Alien
- Flip Mark - Kenny Benjamin
- Ray Kellogg - Detective
- Marion Ross - Agnes Benjamin
- Joyce Van Patten - Rhea Cashman
- David Opatoshu - Ralph Cashman
- Tim O'Connor - Paul Tanner / Voice in The Enemy's war helmet (voice)
- Tim O'Connor - Major Clint Anderson
- Frank Puglia - Father Fontana
- Robert Fortier - Dr. Paul Pollard
- Robert Fortier - Bert Hamil
- Robert Fortier - Budge
- Bert Freed - Joe Hayden
- Konstantin Shayne - The Gardener
- Konstantin Shayne - Dr. Philip Fletcher
- Ted de Corsia - Randolph E. Branch
- Ted de Corsia - New Sentry
- Glenn Cannon - Teenage Luminos
- Joseph Ruskin - Collins
- Signe Hasso - Laural Marshall
- Barry Russo - Dr. Terrell
- Linda Hutchings - Arlene Schnabel
- Linda Hutchings - The Nurse
- Howard Caine - Leon Chambers
- Vaughn Taylor - Dean Flint
- Vaughn Taylor - Randall Latimer
- William O'Connell - The Creature (as William O'Connel)
- Dabney Coleman - James Custer
- Dabney Coleman - Lt. Rupert Lawrence Howard (uncredited)
- Dabney Coleman - Dr. Williams
- Michael Ansara - Qarlo Clobregnny
- Henry Brandon - General Crawford
- Dabbs Greer - E. F. Larkin
- Dabbs Greer - Mr. Bishop
- Ralph Hart - Loren Kagan
- Barbara Rush - Leonora Edmond
- Jill Hill - Toni Kagan
- Allen Jaffe - The Enemy
- Lloyd Gough - Gen. Claiborne
- Malachi Throne - Dr. Mike
- Catherine McLeod - Abby Kagan
- Dean Harens - Medical Technician
- Rudy Solari - Griffin
- Rudy Solari - Captain Buckley
- Lawrence Montaigne - Construction
- Gail Kobe - Janet
- Gail Kobe - Janet Doweling
- Julian Burton - Reporter #1
- Joan Freeman - Elizabeth Dunn
- Parley Baer - Dr. Bernard Stone
- Crahan Denton - Sheriff Simon Stakefield
- Crahan Denton - Dr. Matthew James
- Leonard Nimoy - Konig
- Leonard Nimoy - Judson Ellis
- Keith Andes - Dr. Peter Wayne
- Paul Sorensen - Grayson
- Curt Conway - Franklin Karlin
- Curt Conway - Dr. Philip Mendl
- James Doohan - Police Lt. Branch
- Jack Wilson - Sergeant Jackson
- Mary Gregory - Apartment Manager
- Barbara Wilkin - Susan Wayne
- Peter Duryea - Morrow
- Arlene Martel - Consuelo Biros
- Owen McGiveney - Night Watchman (as Owen McGivney)
- Abraham Sofaer - Arch
- Troy Melton - Det. Sgt. Alger
- Rex Holman - Battle
- Joe Maross - General Winston
- Steve Harris - Breech
- Ted Knight - Mr. Jerome
- Chris Alcaide - Colonel Danvers
- Anthony Costello - Lieutenant Bowman
- Bob Johnson - The Senator (voice)
- Bob Johnson - Project Adonis Intercom Announcer (voice) (uncredited)
- Robert DoQui - Lieutenant Johnson
- Paul Lukather - Official
- Paul Lukather - Nichols
- Michael T. Mikler - Captain Thomas
- Sara Shane - Ethel Meridith
- Marianna Hill - Nina
- Peter Haskell - Peter Jellicoe
- Robert Sorrells - Fred
- Hugh Sanders - Sheriff Barclay
- Mary Jackson - Mrs. McCrae
- Peter Brocco - Professor 'Doc' Link
- Ken Drake - The Judge
- James B. Sikking - Botany
- James B. Sikking - Orderly
- Christine Matchett - Evie
- Robert Nelson - The Surgeon
- William Wintersole - Prof. Andrew Whitsett
- Donald Harron - Art Harris
- Steve Ihnat - Lt. Minns
- Dee Pollock - Francis Hadley, P.F.C.
- James Frawley - Robert Renaldo
- Simon Prescott - The Guard
- Jan Shutan - Mrs. Subiron
- Joanne Stewart - Miss Steen
- Leon Askin - Shop Superintendent
- Paulle Clark - Second Nurse
- Warren Stevens - Eric Plummer
- Kim Hector - Johnny Subiron
- Constance Towers - Laura James
- Mike Lane - The Megasoid
- Steven Geray - Jerichau
- Alan Gifford - The Guide
- Jeffrey Stone - The Cop
- Jonathan Hole - The Pedestrian
- Ivy Bethune - Miss Thorsen
- Charles H. Radilak - Michael Lint
- Sean McClory - Karl Emmet
- Larry Ward - Keith Ellis
- Morgan Brittany - Minerva Gordon
- Morgan Brittany - Minerva
- Elizabeth Perry - Jennifer Barham
- Stephen Joyce - Captain Branson
- Anthony Eisley - Col. Alex Barham
- Douglas Kennedy - Gen. Daniel Pettit
- Shary Marshall - Maggie O'Hara
- Sandy Kenyon - Henry Craif
- Martin Kosleck - Dr. Leo Hausner
- Wesley Addy - Dr. Rahm
- Peter Hansen - Major Locke
- Robert Chadwick - Guard
- Read Morgan - Adam Lunk
- John Caper Jr. - Adam Link (voice)
- Leslie Stevens
- Sam White
- Lou Morheim
Robert Duvall hace el papel de Adam Ballard.
Gladys Cooper interpreta a Mrs. Palmer en la serie.
Ted de Corsia interpreta a Randolph E. Branch en la serie.
Vaughn Taylor interpreta a Dean Flint en la serie.
John Marley interpreta a Dr. Sigmund Hindeman en la serie.
Martin Landau interpreta a Richard Bellero en la serie.
Edward Platt interpreta a David Hunt (as Edward C. Platt) en la serie.
Miriam Hopkins hace el papel de Mary Kry.
Leonard Stone interpreta a Dr. Phillip Gainer en la serie.
Michael Higgins hace el papel de Dr. Thomas Kellander.
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