Gen. Leslie Groves

J. Robert Oppenheimer

Leo Szilard

James F. Byrnes

President Harry S. Truman

Gen. George Marshall

Henry Stimson

Dr. Arthur Compton

Franklin D. Roosevelt

Kitty Oppenheimer

Colonel Pash

Edward Teller

Seth

Dwight D. Eisenhower

Groves's Commanding Officer

Ralph A. Bard

Enrico Fermi

Trude Weiss

Colonel Nichols

Eugen Wigner

Albert Einstein

Richard Feynman

Hans Bethe

Samuel Goudsmit

Klaus Fuchs

Lord Rutherford

Haakon Chevalier

Michael Polanyi

Ilse Polanyi

Colonel Lansdale

Robert Wilson

Mrs. Wisnovsky

Jack Wisnovsky

Shapp

Mrs. Trowbridge

Robert Serber

Ed Condon

William 'Deke' Parsons

Captain DeSilva

T. O. Jones

James Tuck

Harry Gold

Army Major

Fleischmann

Haagen Kistiakowski

George Kistiakowski

Kenneth Bainbridge

William D. Leahy

Paul Tibbets

Air Force Guard

Chemist

Alfred MacCormack

Army General

John J. McCloy

Allan Dulles

London Pedestrian

British Jr. Officer

British Sr. Officer

Alexander Sachs

General Pa Watson

Los Alamo Scientist (uncredited)

Scientist (uncredited)

Leo Szilard's Prime Scientist (uncredited)