Hugh Marlowe as Dr. Russell A. Marvin

Joan Taylor as Carol Marvin

Donald Curtis as Maj. Huglin

Morris Ankrum as Brig. Gen. John Hanley

Thomas Browne Henry as Vice Adm. Enright

Grandon Rhodes as Gen. Edmunds

John Zaremba as Prof. Kanter

Paul Frees as Alien (voice)

Frank Wilcox as Alfred Cassidy

Sydney Mason as Frank the garage man

Larry J. Blake as Motorcycle cop

Bert Stevens as Air Intelligence Command Officer

Forbes Murray as Military Officer at Experiment

Arthur Tovey as Officer/Civilian at Military Conference

Alan Wells as Major Kimberly

Nicky Blair as Military Officer at Experiment (uncredited)

Jimmy Cross as Military Messenger (uncredited)

Jack Deery as Military Officer (uncredited)

Charles Evans as Dr. Alberts (uncredited)

Raoul Freeman as Military Official (uncredited)

Paul Frees as Alien (voice) (uncredited)

James Gonzalez as Military Official (uncredited)

Duke Green as Minor Role (uncredited)

Ed Haskett as Military Official (uncredited)

Clark Howat as Sgt. Nash (uncredited)

Harry Lauter as Cutting - Generator Technician (uncredited)

Don Marlowe as Minor Role (uncredited)

William Meader as Worker in Control Tower (uncredited)

Sol Murgi as Plane Passenger (uncredited)

Forbes Murray as Military Officer at Experiment (uncredited)

William J. O'Brien as Radio Listener (archive footage) (uncredited)

Murray Pollack as Plane Passenger (uncredited)

Mike Ragan as Tank Commander (uncredited)

Alan Reynolds as Maj. Kimberly (uncredited)

Dale Van Sickel as Man Crushed Beneath Wall (uncredited)

Guy Way as Civilian Observing Saucer Attack (uncredited)

Bob Whitney as Plane Passenger (uncredited)

Frank Wilcox as Alfred Cassidy (uncredited)

Beal Wong as Chinese Radio Listener (uncredited)

William Woodson as Narrator (voice) (uncredited)