Mark Ruffalo (Inspector David Toschi)
Jake Gyllenhaal (Robert Graysmith)
Robert Downey, Jr. (Paul Avery)
Anthony Edwards (Inspector William Armstrong)
Brian Cox (Melvin Belli)
Clea Duvall (Linda del Buono)
Barry Livingston (Copyeditor)
John Lacy (Zodiac 4)
Donal Logue (Ken Narlow)
John Carroll Lynch (Arthur Leigh Allen)
Dermot Mulroney (Captain Marty Lee)
Ed Setrakian (Al Hyman)
Chloë Sevigny (Melanie)
John Getz (Templeton Peck)
John Terry (Charles Thieriot)
Candy Clark (Carol Fisher)
Adam Goldberg (Duffy Jennings)
James LeGros (Officer George Bawart)
Doan Ly (Belli's Housekeeper)
Pell James (Cecelia Shepherd)
Lee Norris (Mike Mageau-Young)
Bob Stephenson (Zodiac 3)
Jason Wiles (Lab Tech Dagitz)
Joel Bissonnette (Inspector Kracke)
Richmond Arquette (Zodiac 1 & 2)
Charles Fleischer (Bob Vaughn)
Zach Grenier (Mel Nicolai)
Philip Baker Hall (Sherwood Morrill)
Elias Koteas (Sgt. Jack Mulanax)