George Murphy
— Inspector James 'Jim' Belden
Finlay Currie
— Professor Albert Kafer
Virginia Gilmore
— Millie / Teresa Zalenko
Karl Stepanek
— Alexi Laschenkov / Gregory Anders
Louisa Horton
— Mrs. Elaine Wilben
Peter Capell
— Chris Zalenko / Gino
Bruno Wick
— Luther Danzig
Vilma Kurer
— Mrs. Rita Foss
Ann Thomas
— Philadelphia Suspect
George Roy Hill
— Nicholas Wilben
Karl Weber
— FBI Agent Charlie Reynolds
Michael Garrett
— Michael Dorndoff aka Frank Torrance
Lotte Palfi Andor
— Anna Kafer
Ernest Graves
— Robert Martin
Robert Carroll
— Boldany
Wolfgang Zilzer
— August Helmuth
Bradford Hatton
— Harry Mason
Rosemary Pettit
— Mrs. Martin
Eva Condon
— Landlady
Steve Mitchell
— Samson
John Farrell
— Taxi Driver (uncredited)
Nancy Hale
— Mrs. Belden (uncredited)
Westbrook Van Voorhis
— Narrator (voice) (uncredited)
Mary Young
— Old Lady (uncredited)
Suzanne Moulton
— Sherry Belden (uncredited)