Vic Perrin
Control Voice (voice) (uncredited)

Ivan Dixon
Sgt. James Conover / Major Harold Giles

John Hoyt
Bifrost Alien / Emmett Balfour / Professor Hebbel

Robert Culp
Trent / Allen Leighton / Paul Cameron

Robert Duvall
Adam Ballard / Louis Mace

Willard Sage
Coulter / Chief of Staff / First Reporter

Jason Wingreen
Fred Severn / Bill Turner / Coroner Leland

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

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

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

Robert Fortier
Dr. Paul Pollard / Budge / Bert Hamil

Ted de Corsia
Randolph E. Branch / New Sentry

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

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

Philip Abbott
Ben Fields / Lincoln Russell

Henry Silva
General Juan Mercurio / Chino Rivera

Kent Smith
Dr. Block / Aabel

Janos Prohaska
Thetan (uncredited) / The Mikie

William Stevens
Navigator Dexter / Policeman

Martin Landau
Andro / Richard Bellero