Jake Gyllenhaal
as Robert Graysmith

Mark Ruffalo
as David Toschi

Anthony Edwards
as William Armstrong

Robert Downey Jr.
as Paul Avery

Chloë Sevigny
as Melanie

Elias Koteas
as Jack Mulanax

John Carroll Lynch
as Arthur Leigh Allen

Brian Cox
as Melvin Belli

Dermot Mulroney
as Martin Lee

Charles Fleischer
as Bob Vaughn

Zach Grenier
as Mel Nicolai

Philip Baker Hall
as Sherwood Morrill

James Le Gros
as George Bawart

Donal Logue
as Ken Narlow

Richmond Arquette
as Zodiac 1 / Zodiac 2

Bob Stephenson
as Zodiac 3

John Lacy
as Zodiac 4

Ed Setrakian
as Al Hyman

John Getz
as Templeton Peck

John Terry
as Charles Thieriot

Candy Clark
as Carol Fisher

June Diane Raphael
as Mrs. Toschi

Ciara Hughes
as Darlene Ferrin

Lee Norris
as Young Mike Mageau

Patrick Scott Lewis
as Bryan Hartnell

Pell James
as Cecilia Shepard

David Lee Smith
as Father

Jason Wiles
as Lab Tech Dagitz

Charles Schneider
as Cabbie / Paul Stine

James Carraway
as Shorty

Tom Verica
as Jim Dunbar

Jimmi Simpson
as Mike Mageau

Doan Ly
as Belli's Housekeeper

Karina Logue
as Woman

Joel Bissonnette
as Inspector Kracke

John Mahon
as Riverside Captain

Matt Winston
as John Allen

Jules Bruff
as Catherine Allen

John Ennis
as Terry Pascoe

J. Patrick McCormack
as Police Commissioner

Adam Goldberg
as Duffy Jennings

Clea DuVall
as Linda del Buono

Paul Schulze
as Sandy Panzarella

Adam Trese
as Detective #1

Penny Wallace
as Mulanax's Secretary

John Hemphill
as Donald Cheney

Michel Francoeur
as Man on Marquee

Thomas Kopache
as Copy Editor #1

Barry Livingston
as Copy Editor #3

Christopher John Fields
as Copy Editor #4

Jack Samson
as Young David Graysmith

Micah Sauers
as David Graysmith

Zachary Sauers
as Aaron Graysmith

Jessica Baltutis
as Margot Graysmith

Peter Quartaroli
as Patrolman Fouke

Geoff Callan
as Patrolman Zelms

Michael Hungerford
as Truck Driver

Anna Katarina
as Society Woman

E.P. McKnight
as Informant (Hall of Justice A)

Betty Murphy
as Informant

Jeff Daniel Phillips
as Informant

Shane Woodson
as Informant

Cookie Crawford
as Informant #8

Cassius M. Willis
as Uniform Cop

Judith Drake
as Florence Douglas

Phoebe Holston
as Prison Guard

Marty Lodge
as Pinole Foreman

Cooper Thornton
as Internal Affairs Agent #1

Cazimir Milostan
as Internal Affairs Agent #2

Charlotte Ferguson
as Kathleen Johns' baby

Phoebe Ferguson
as Kathleen Johns' baby

Kacey Malmsten
as Kathleen Johns' baby

Karly Malmsten
as Kathleen Johns' baby

JD Cullum
as Director

Stanley B. Herman
as Staff Editor

Roy Lee Jones
as Distraught African American Male

Michael Rose
as Navy Project Leader

James Joseph O'Neil
as DOD Project Leader

Rod Damer
as FBI Codebreaker

John Sarno
as Donald Harden

Gloria Grant
as Bettye Harden

Brett Rickaby
as Detective Roy

M.F. Bernier
as Detective #2

Carter Evans
as Newscaster #2

Bill Seward
as TV News Anchor

Dave Nemeth
as Newscaster #4

Ted Garcia
as Anchorman #1

Greg Wolf
as Interviewer #1

Barry LeBrock
as Reporter

Ione Skye
as Kathleen Johns (uncredited)

David Winston Barge
as Vallejo Desk Ofiicer (uncredited)

Hayati Akbas
as Hardware Store Customer (uncredited)

Brad Carr
as Camera Man (uncredited)

Mitchell Fink
as Reporter (uncredited)

Erica Ford
as Traveler (uncredited)

Tish Hicks
as Informant (uncredited)

Danielle McKee
as Prisoner (uncredited)

Derris Nile
as Office Worker (uncredited)

Carmen Plumb
as Janie (uncredited)

Callie Thompson
as Child Witness (uncredited)

Bud Davis
as Patrolman (uncredited)