Vic Perrin
Control Voice (voice) (uncredited)

Ivan Dixon
Sgt. James Conover / Major Harold Giles

John Hoyt
Bifrost Alien / Professor Hebbel / Emmett Balfour

Robert Culp
Trent / Paul Cameron / Allen Leighton

Robert Duvall
Adam Ballard / Louis Mace

Willard Sage
Coulter / First Reporter / Chief of Staff

Jason Wingreen
Fred Severn / Coroner Leland / Bill Turner

Edward Platt
Dean Radcliffe / Mr. Terrence / David Hunt (as Edward C. Platt)

Douglas Henderson
Dr. Paul Fredericks (uncredited) / Dr. Tillyard / Detective Lt. Runyan

Ben Wright
Philip Exeter Dundee / Elder of Luminos / Gen. Benton

Robert Fortier
Bert Hamil / Budge / Dr. Paul Pollard

Ted de Corsia
Randolph E. Branch / New Sentry

Dabney Coleman
James Custer / Dr. Williams / Lt. Rupert Lawrence Howard (uncredited)

Philip Abbott
Lincoln Russell / Ben Fields

David Frankham
Harvey Kry Jr. / Capt. Terrence Ralph Brookman

Henry Silva
General Juan Mercurio / Chino Rivera

Kent Smith
Aabel / Dr. Block

William Stevens
Policeman / Navigator Dexter

Janos Prohaska
Thetan (uncredited) / The Mikie

Martin Landau
Andro / Richard Bellero