Brian Dennehy

Gen. Leslie Groves

David Strathairn

J. Robert Oppenheimer

Michael Tucker

Leo Szilard

Hume Cronyn

James F. Byrnes

Richard Dysart

President Harry S. Truman

Hal Holbrook

Gen. George Marshall

Barnard Hughes

Henry Stimson

John McMartin

Dr. Arthur Compton

David Ogden Stiers

Franklin D. Roosevelt

Anne Twomey

Kitty Oppenheimer

Lawrence Dane

Ron Frazier

Colonel Pash

Olek Krupa

Edward Teller

Bernie McInerney

John Pielmeier

Seth

Ken Pogue

Dwight D. Eisenhower

Gary Reineke

Groves's Commanding Officer

Alan Scarfe

John Seitz

Ralph A. Bard

Tony Shalhoub

Enrico Fermi

Ester Spitz

Trude Weiss

Ron White

Colonel Nichols

Stephan Balint

Eugen Wigner

Peter Boretski

Albert Einstein

Patrick Breen

Richard Feynman

Vlasta Vrana

Hans Bethe

George Popovich

Samuel Goudsmit

Lorne Brass

Klaus Fuchs

Michael Sinelnikoff

Lord Rutherford

Graham Haley

Haakon Chevalier

Kliment Denchev

Michael Polanyi

Diana Fajrajsl

Ilse Polanyi

Timothy Webber

Colonel Lansdale

Harry Standjofski

Robert Wilson

Lia Sargent

Mrs. Wisnovsky

David Gow

Jack Wisnovsky

Michael Farrell

Shapp

Isabelle Mejias

Mrs. Trowbridge

Jonathan Wise

Robert Serber

George R. Robertson

Ed Condon

Dee McCafferty

William 'Deke' Parsons

Tom Butler

Captain DeSilva

Robert Morelli

T. O. Jones

Jon Baggaley

James Tuck

David Bolt

Harry Gold

David Ferry

Army Major

Heinar Pillar

Fleischmann

François Klanfer

Haagen Kistiakowski

Joel Miller

George Kistiakowski

Michael J. Reynolds

Kenneth Bainbridge

Ed McGibbon

William D. Leahy

Nicholas Kilbertus

Paul Tibbets

Joe Cazalet

Air Force Guard

Scott Thomson

Chemist

Bob Clout

Alfred MacCormack

Roland Hewgill

Army General

Stan Gibbons

John J. McCloy

Terrence Labrosse

Allan Dulles

Delise Alison

London Pedestrian

Alex Guard

British Jr. Officer

Philip Spensley

British Sr. Officer

Frederick Neumann

Alexander Sachs

Norris Domingue

General Pa Watson

Steve Barry

Los Alamo Scientist (uncredited)

Arthur Holden

Scientist (uncredited)

Romano Orzari

Leo Szilard's Prime Scientist (uncredited)