Previous research suggests that being imitated by an adult increases the social behaviors of children with an autism spectrum disorder (ASD). In the current study, we examined whether familiarity with the imitating social partner modulates this effect. Ten children with ASD and their mothers participated. The children's social behaviors were observed prior to and following a 3-min period in which an adult social partner imitated everything they did. In one condition the partner was the child's mother, and in the other condition the partner was an unfamiliar experimenter. The results revealed significant increases in distal social behaviors (gazes toward the adult, vocalizing) following imitation by both partners. There was a significantly greater increase in proximal social behaviors (including approach, being physically close, and touching) and a greater decrease in playing alone when the imitator was the child's mother as opposed to the experimenter. The findings suggest that the experience of being imitated creates an atmosphere of mutuality and rapport between children with ASD and their social partners, which increases their sociability even in interactions with already familiar adults.