Brian Donlevy

Character: Maj. Gen. Leslie R. Groves

Robert Walker

Character: Col. Jeff Nixon

Tom Drake

Character: Matt Cochran

Beverly Tyler

Character: Anne Cochran

Hume Cronyn

Character: Dr. J. Robert Oppenheimer

Audrey Totter

Character: Jean O'Leary

Hurd Hatfield

Character: Dr. John Wyatt

Joseph Calleia

Character: Dr. Enrico Fermi

Godfrey Tearle

Character: President Franklin D. Roosevelt

Victor Francen

Character: Dr. Marre

Richard Haydn

Character: Dr. Chisholm

Jonathan Hale

Character: Dr. Vannevar Bush

John Litel

Character: K.T. Keller

Henry O'Neill

Character: Gen. Thomas F. Farrell

Warner Anderson

Character: Capt. William S. Parsons

Ellen Ross

Character: Waitress

Bobby Jordan

Character: Radioman on Tinian Receiving A-Bomb Message

Jerry Schumacher

Character: Nazi Officer