Roddy McDowall
Cornelius

Kim Hunter
Dr. Zira

Bradford Dillman
Dr. Lewis Dixon

Natalie Trundy
Dr. Stephanie 'Stevie' Branton

Eric Braeden
Dr. Otto Hasslein

William Windom
The President

Sal Mineo
Dr. Milo

Albert Salmi
E-1

Jason Evers
E-2

John Randolph
Committee Chairman

Harry Lauter
General Winthrop

M. Emmet Walsh
Aide

Roy Glenn
Lawyer (as Roy E. Glenn Sr.)

Peter Forster
Cardinal

Norman Burton
Army Officer

William Woodson
Naval Officer

Tom Lowell
Orderly

Donald Elson
Curator

James Bacon
General Faulkner

Ricardo Montalban
Armando

John Alderman
Marine Corporal (uncredited)

Alan Baxter
General (uncredited)

Jack Berle
Secretary of State (uncredited)

Sam Chew Jr.
Undetermined (uncredited)

Walker Edmiston
Talking Baby Chimp (uncredited) (voice)

Bob Harks
Aide (uncredited)

Linda Harrison
Nova (archive footage) (uncredited)

Elizabeth Harrower
Reporter at Hotel (uncredited)

Shep Houghton
Bystander (uncredited)

Robert Nichols
Reporter (uncredited)

Hank Robinson
Reporter (uncredited)

James B. Sikking
Control Room Officer (uncredited)

Arthur Tovey
Official at Briefing (uncredited)

William Tregoe
Hotel Receptionist (uncredited)

William Beckley
British News Anchor (uncredited)