Roddy McDowall as Cornelius

Kim Hunter as Dr. Zira

Bradford Dillman as Dr. Lewis Dixon

Natalie Trundy as Dr. Stephanie 'Stevie' Branton

Eric Braeden as Dr. Otto Hasslein

William Windom as The President

Sal Mineo as Dr. Milo

Albert Salmi as E-1

Jason Evers as E-2

John Randolph as Committee Chairman

Harry Lauter as General Winthrop

M. Emmet Walsh as Aide

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

Peter Forster as Cardinal

Norman Burton as Army Officer

William Woodson as Naval Officer

Tom Lowell as Orderly

Gene Whittington as Marine Captain

Donald Elson as Curator

Bill Bonds as TV Newscaster

Army Archerd as Referee

James Bacon as General Faulkner

Ricardo Montalban as Armando

John Alderman as Marine Corporal (uncredited)

Alan Baxter as General (uncredited)

Jack Berle as Secretary of State (uncredited)

Paul Bradley as Official at Briefing (uncredited)

Karl Bruck as German Newscaster (uncredited)

Sam Chew Jr. as Undetermined (uncredited)

Robert Cole as Reporter (uncredited)

Walker Edmiston as Talking Baby Chimp (uncredited) (voice)

Sig Frohlich as Spectator (uncredited)

James W. Gavin as Helicopter Pilot (uncredited)

George Golden as Official at Briefing (uncredited)

James Gonzalez as Official at Briefing (uncredited)

Joe Gray as Bodyguard (uncredited)

Bob Harks as Aide (uncredited)

Linda Harrison as Nova (archive footage) (uncredited)

Elizabeth Harrower as Reporter at Hotel (uncredited)

Shep Houghton as Bystander (uncredited)

Joseph La Cava as Waiter (uncredited)

Robert Nichols as Reporter (uncredited)

Ron Pinkard as Undetermined (uncredited)

Janos Prohaska as Heloise (uncredited)

Tony Regan as Reporter at Hotel (uncredited)

Stephen Roberts as Gen. Brody (uncredited)

Hank Robinson as Reporter (uncredited)

James B. Sikking as Control Room Officer (uncredited)

Jack Slate as Reporter (uncredited)

Arthur Tovey as Official at Briefing (uncredited)

William Tregoe as Hotel Receptionist (uncredited)

William Beckley as British News Anchor (uncredited)