Our study assessed whether the peer environment in kindergarten and first grade affected student learning following an early mathematics intervention. We leveraged longitudinal data from a cluster-RCT to examine whether math achievement in kindergarten (n = 1,218) and first grade (n = 1,126) was affected by either the share of high-achieving classmates or the proportion of classroom peers who received a preschool math curriculum intervention. Analyses indicated that exposure to treated peers in first grade, but not kindergarten, was significantly associated with small gains in end-of-year achievement. Some analyses also suggested that average peer math achievement was positively related to children's kindergarten and first-grade achievement across conditions, though these results were less robust. We did not find consistent evidence to suggest that the proportion of treated peers coincided with better teaching practices. Taken together, these findings suggest that classroom peer effects may play only a limited role in sustaining early intervention effects. (author abstract)