The intent of this study was to examine the development of peer interaction in low-income Mexican-heritage families in the United States. Eighty-eight children (44 girls) were observed and mothers interviewed when children were 14, 24, 36, and 54 months old. We used the Attachment Q-Set (Waters, 1990), the Peer Play Scale (Howes & Matheson, 1992), and the Children's Behavior Ratings (Ladd, 1999). Complex peer play, particularly social pretend play, was relatively infrequent for all children. Children with more secure mother-child attachment relationships also had more concurrent complex play and more rapid growth of complex play. Children who engaged in more social pretend play at 54 months were more likely to engage in more language interaction when younger.