Brian Dennehy

as Gen. Leslie Groves

David Strathairn

as J. Robert Oppenheimer

Michael Tucker

as Leo Szilard

Hume Cronyn

as James F. Byrnes

Richard Dysart

as President Harry S. Truman

Hal Holbrook

as Gen. George Marshall

Barnard Hughes

as Henry Stimson

John McMartin

as Dr. Arthur Compton

David Ogden Stiers

as Franklin D. Roosevelt

Anne Twomey

as Kitty Oppenheimer

Lawrence Dane

Ron Frazier

as Colonel Pash

Olek Krupa

as Edward Teller

Bernie McInerney

John Pielmeier

as Seth

Ken Pogue

as Dwight D. Eisenhower

Gary Reineke

as Groves's Commanding Officer

Alan Scarfe

John Seitz

as Ralph A. Bard

Tony Shalhoub

as Enrico Fermi

Ester Spitz

as Trude Weiss

Ron White

as Colonel Nichols

Stephan Balint

as Eugen Wigner

Peter Boretski

as Albert Einstein

Patrick Breen

as Richard Feynman

Vlasta Vrana

as Hans Bethe

George Popovich

as Samuel Goudsmit

Lorne Brass

as Klaus Fuchs

Michael Sinelnikoff

as Lord Rutherford

Graham Haley

as Haakon Chevalier

Kliment Denchev

as Michael Polanyi

Diana Fajrajsl

as Ilse Polanyi

Timothy Webber

as Colonel Lansdale

Harry Standjofski

as Robert Wilson

Lia Sargent

as Mrs. Wisnovsky

David Gow

as Jack Wisnovsky

Michael Farrell

as Shapp

Isabelle Mejias

as Mrs. Trowbridge

Jonathan Wise

as Robert Serber

George R. Robertson

as Ed Condon

Dee McCafferty

as William 'Deke' Parsons

Tom Butler

as Captain DeSilva

Robert Morelli

as T. O. Jones

Jon Baggaley

as James Tuck

David Bolt

as Harry Gold

David Ferry

as Army Major

Heinar Piller

as Fleischmann

François Klanfer

as Haagen Kistiakowski

Joel Miller

as George Kistiakowski

Michael J. Reynolds

as Kenneth Bainbridge

Ed McGibbon

as William D. Leahy

Nicholas Kilbertus

as Paul Tibbets

Joe Cazalet

as Air Force Guard

Scott Thomson

as Chemist

Bob Clout

as Alfred MacCormack

Roland Hewgill

as Army General

Stan Gibbons

as John J. McCloy

Terrence Labrosse

as Allan Dulles

Delise Alison

as London Pedestrian

Alex Guard

as British Jr. Officer

Philip Spensley

as British Sr. Officer

Frederick Neumann

as Alexander Sachs

Norris Domingue

as General Pa Watson

Steve Barry

as Los Alamo Scientist (uncredited)

Arthur Holden

as Scientist (uncredited)

Romano Orzari

as Leo Szilard's Prime Scientist (uncredited)