regulars:

Frankie Muniz as Malcolm
Jane Kaczmarek as Lois, Malcolm's mother
Bryan Cranston as Hal, Malcolm's father
Christopher Kennedy Masterson as Francis, Malcolm's eldest brother
Justin Berfield as Reese, Malcolm's older brother
Erik Per Sullivan as Dewey, Malcolm's younger brother
Catherine Lloyd Burns as Caroline, Malcolm's teacher
 

recurring characters:
Craig Lamar Traylor as Stevie Kenarban, Malcolm's friend
Karim Prince as Stanley, Francis' cohort
David Higgins as Craig, Lois' co-worker
Paul Willson as Ed, the neighbor
Landry Allbright as Julie