Gen. Leslie Groves

Brian Dennehy

J. Robert Oppenheimer

David Strathairn

Leo Szilard

Michael Tucker

James F. Byrnes

Hume Cronyn

President Harry S. Truman

Richard Dysart

Gen. George Marshall

Hal Holbrook

Secretary Stimson

Barnard Hughes

Dr. Arthur Compton

John McMartin

Franklin D. Roosevelt

David Ogden Stiers

Kitty Oppenheimer

Anne Twomey

Enrico Fermi

Tony Shalhoub

Colonel Pash

Ron Frazier

Lawrence Dane

Bernie McInerney

Edward Teller

Olek Krupa

Dwight D. Eisenhower

Ken Pogue

Groves's Commanding Officer

Gary Reineke

Albert Einstein

Peter Boretski

Seth

John Pielmeier

Alan Scarfe

Eugen Wigner

Stefan Balint

Ralph A. Bard

John Seitz

Colonel Nichols

Ron White

Richard Feynman

Patrick Breen

Trude Weiss

Ester Spitz

Hans Bethe

Vlasta Vrana

Samuel Goudsmit

George Popovich

Eugen Wigner

Stephan Balint

Klaus Fuchs

Lorne Brass

Lord Rutherford

Michael Sinelnikoff

Haakon Chevalier

Graham Haley

Colonel Lansdale

Timothy Webber

Jack Wisnovsky

David Gow

Mrs. Trowbridge

Isabelle Mejias

Robert Serber

Jonathan Wise

Ed Condon

George R. Robertson

Michael Polanyi

Kliment Denchev

Captain DeSilva

Tom Butler

Ilse Polanyi

Diana Fajrajsl

T. O. Jones

Robert Morelli

James Tuck

Jon Baggaley

Robert Wilson

Harry Standjofski

Harry Gold

David Bolt

Mrs. Wisnovsky

Lia Sargent

Army Major

David Ferry

Haagen

Francois Klanfer

Shapp

Michael Farrell

Kenneth Bainbridge

Michael J. Reynolds

Air Force Guard

Joe Cazalet

Chemist

Scott Thomson

Alfred MacCormack

Bob Clout

William 'Deke' Parsons

Dee McCafferty

Army General

Roland Hewgill

Allan Dulles

Terence Labrosse

British Sr. Officer

Philip Spensley

Alexander Sachs

Frederick Neumann

General Pa Watson

Norris Domingue

Fleischmann

Heinar Piller

Fleischmann

Heinar Pillar

George Kistiakowski

Joel Miller

William D. Leahy

Ed McGibbon

Paul Tibbets

Nicholas Kilbertus

John J. McCloy

Stan Gibbons

Allan Dulles

Terrence Labrosse

British Jr. Officer

Alex Guard

Los Alamo Scientist (uncredited)

Steve Barry

Scientist (uncredited)

Arthur Holden

Leo Szilard's Prime Scientist (uncredited)

Romano Orzari