Vic Perrin
Control Voice (voice) (uncredited)

Ivan Dixon
Sgt. James Conover / Major Harold Giles

John Hoyt
Professor Hebbel / Bifrost Alien / Emmett Balfour

Robert Culp
Trent / Allen Leighton / Paul Cameron

Robert Duvall
Adam Ballard / Louis Mace

Willard Sage
First Reporter / Chief of Staff / Coulter

Jason Wingreen
Coroner Leland / Fred Severn / Bill Turner

Edward Platt
David Hunt (as Edward C. Platt) / Mr. Terrence / 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
Budge / Dr. Paul Pollard / Bert Hamil

Ted de Corsia
Randolph E. Branch / New Sentry

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

Philip Abbott
Ben Fields / Lincoln Russell

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

Henry Silva
Chino Rivera / General Juan Mercurio

Kent Smith
Dr. Block / Aabel

Janos Prohaska
The Mikie / Thetan (uncredited)

William Stevens
Navigator Dexter / Policeman

Geraldine Brooks
Yvette Leighton / Ann Barton