Seth Green

Milo (performance capture)

Seth Robert Dusky

Milo (voice) (as Seth Dusky)

Joan Cusack

Mom (voice)

Dan Fogler

Gribble (voice)

Breckin Meyer

Alien (voice)

Elisabeth Harnois

Ki (voice)

Tom Everett Scott

Milo's Dad (voice)

Mindy Sterling

Supervisor (voice)

Julene Renee

Martian (voice)

Raymond Ochoa

Martian Hatchling (voice)

Robert Ochoa

Martian Hatchling (voice)

Ryan Ochoa

Martian Hatchling (voice)

Amber Gainey Meade

Martian

Dee Bradley Baker

Two-Cat (uncredited)