John Carradine as Dr. Sigmund Walters

Evelyn Ankers as Beth Colman

Milburn Stone as Fred Mason

Lloyd Corrigan as John Whipple

Fay Helm as Nurse Strand

Martha Vickers as Dorothy Colman

Acquanetta as Paula Dupree, the Ape Woman

Vince Barnett as Curly

Paul Fix as Gruen

Fred Aldrich as Roustabout (uncredited)

Clyde Beatty as Fred Mason (in long shots) (archive footage / uncredited)

Turhan Bey as End Narrator (voice)(uncredited)

Eddie Borden as Dock Spectator (uncredited)

Ray Corrigan as Cheela the Gorilla (uncredited)

Russell Custer as Curley (uncredited)

Fern Emmett as Beth's Murdered Neighbor (uncredited)

Virginia Engels as Trapeze Artist (uncredited)

Alexander Gill as Waiter (uncredited)

Gus Glassmire as Coroner (uncredited)

Joel Goodkind as Small Boy (uncredited)

William Gould as Sheriff (uncredited)

Al Haskell as Al (uncredited)

Harry Holman as Dock Ticket Office Clerk

Tom London as Ship's Captain (archive footage)

Charles McAvoy as Cop (uncredited)

Frank Mitchell as Handler (uncredited)

Charles Morton as Dock Worker (uncredited)

Edward Peil Sr. as Jake - Handler (uncredited)

Stanley Price as Man on Dock (uncredited)

Joey Ray as Attendant (uncredited)

Cap Somers as Roustabout (uncredited)

Ray Walker as Ringmaster (uncredited)

Anthony Warde as Tony - Handler (uncredited)

Grant Withers as Veterinarian (uncredited)