Ivan Dixon
Sgt. James Conover / Major Harold Giles

John Hoyt
Professor Hebbel / Emmett Balfour / Bifrost Alien

Robert Culp
Paul Cameron / Trent / Allen Leighton

Robert Duvall
Adam Ballard / Louis Mace

Willard Sage
Chief of Staff / Coulter / First Reporter

Jason Wingreen
Fred Severn / Coroner Leland / Bill Turner

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

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

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

Robert Fortier
Budge / Dr. Paul Pollard / Bert Hamil

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
Lincoln Russell / Ben Fields

Henry Silva
General Juan Mercurio / Chino Rivera

Kent Smith
Aabel / Dr. Block

Janos Prohaska
Thetan (uncredited) / The Mikie

William Stevens
Policeman / Navigator Dexter

Martin Landau
Richard Bellero / Andro

Geraldine Brooks
Ann Barton / Yvette Leighton