Vic Perrin
Control Voice (voice) (uncredited)

Ivan Dixon
Sgt. James Conover / Major Harold Giles

John Hoyt
Professor Hebbel / Bifrost Alien / Emmett Balfour

Robert Culp
Paul Cameron / Allen Leighton / Trent

Robert Duvall
Adam Ballard / Louis Mace

Willard Sage
Chief of Staff / Coulter / First Reporter

Jason Wingreen
Coroner Leland / Bill Turner / Fred Severn

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

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

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

Robert Fortier
Bert Hamil / Dr. Paul Pollard / Budge

Ted de Corsia
Randolph E. Branch / New Sentry

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

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

Philip Abbott
Ben Fields / Lincoln Russell

Henry Silva
Chino Rivera / General Juan Mercurio

Kent Smith
Aabel / Dr. Block

William Stevens
Policeman / Navigator Dexter

Janos Prohaska
The Mikie / Thetan (uncredited)

Martin Landau
Andro / Richard Bellero