Children enter preschool with temperament traits that may shape or be shaped by their social interactions in the peer setting. We collected classroom observational measures of positive emotionality (PE), negative emotionality (NE), effortful control (EC), and peer social play relationships from 2 complete preschool classrooms (N = 53 children) over the course of an entire school year. Using longitudinal social network analysis, we found evidence that children’s traits shaped the formation of play relationships, and that the traits of children’s playmates shaped the subsequent development of children’s own traits. Children who exhibited high levels of NE were less likely to form social play relationships over time. In addition, children were more likely to form play relationships with peers who were similar to their own levels of PE. Over the course of the school year, children’s level of PE and EC changed such that they became more similar to their playmates in levels of these traits. Finally, we observed moderate to strong rank-order stability of behavioral observations of PE, NE, and EC across the school year. Our results provide evidence for the effects of traits on the formation of play relationships, as well as for the role of these play relationships in shaping trait expression over time. (author abstract)