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 / Paul Cameron / Allen Leighton

Robert Duvall
Adam Ballard / Louis Mace

Willard Sage
Coulter / Chief of Staff / First Reporter

Jason Wingreen
Bill Turner / Coroner Leland / Fred Severn

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

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

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

Philip Abbott
Ben Fields / Lincoln Russell

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

Henry Silva
General Juan Mercurio / Chino Rivera

Kent Smith
Aabel / Dr. Block

Janos Prohaska
The Mikie / Thetan (uncredited)

William Stevens
Policeman / Navigator Dexter

Martin Landau
Richard Bellero / Andro