This study examined motor behaviors in a longitudinal cohort of infant siblings of children with autism. Stereotypic movements and postures occurring during standardized observational assessments at 12 and 18 months were coded from videotapes. Participants included eight infant siblings later diagnosed with autism spectrum disorder (ASD), a random sample of nine non-diagnosed siblings, and 15 controls. Videos were coded blind to diagnostic group. At 12 and 18 months the ASD group "arm waved" more frequently and at 18 months, one posture ("hands to ears") was more frequently observed in the ASD and non-diagnosed group compared to the controls. Overall, the siblings subsequently diagnosed with ASD and the comparison groups had considerable overlap in their repertoires of stereotyped behaviors.