Hugh Marlowe

Dr. Russell A. Marvin

Joan Taylor

Carol Marvin

Donald Curtis

Maj. Huglin

Morris Ankrum

Brig. Gen. John Hanley

Thomas Browne Henry

Vice Adm. Enright

Grandon Rhodes

Gen. Edmunds

John Zaremba

Prof. Kanter

Paul Frees

Alien (voice)

Frank Wilcox

Alfred Cassidy

Larry J. Blake

Motorcycle cop

Bert Stevens

Air Intelligence Command Officer

Forbes Murray

Military Officer at Experiment

Arthur Tovey

Officer/Civilian at Military Conference

Nicky Blair

Military Officer at Experiment (uncredited)

Charles Evans

Dr. Alberts (uncredited)

Paul Frees

Alien (voice) (uncredited)

Clark Howat

Sgt. Nash (uncredited)

Harry Lauter

Cutting - Generator Technician (uncredited)

Sol Murgi

Plane Passenger (uncredited)

Forbes Murray

Military Officer at Experiment (uncredited)

Mike Ragan

Tank Commander (uncredited)

Dale Van Sickel

Man Crushed Beneath Wall (uncredited)

Frank Wilcox

Alfred Cassidy (uncredited)

William Woodson

Narrator (voice) (uncredited)