This study examined Vygotsky's theoretical notion that play creates the zone of proximal development by investigating the effects of peer interaction within the context of constructive play with blocks. The authors used the Play Observation Scale (Rubin, 1989) to observe 100 four-year-olds in naturalistic settings. Forty-eight children were selected from the original sample for participation in play sessions in a laboratory setting. Sample selection was based on gender, unfamiliarity, and play level displayed during the sample selection process. Data collected were examined in terms of block play, communication, and peer collaboration. Results of an overall MANOVA conducted for boys and girls found a significant interaction between treatment (play level) and gender. Follow-up MANOVAs were significant for girls, but not for boys. Subsequent univariate tests found significant differences in block play for girls in treatment groups. A pairwise MANOVA found that girls who display fewer play skills engage in more complex play when paired with more skilled or mature peers. Results of a second pairwise MANOVA established that girls who have more complex play skills display even more complex play behavior during play with less-skilled playmates. In addition, they assume a leadership role. Results of this study indicate that the play skills of peers affect the play of preschool girls but not that of preschool boys. These findings have implications for practice for both boys and girls. (Contains 5 tables.)