This article presents elements of an ethnographic study of gendered identities among boys and girls in the early years of primary schooling. Foregrounding group work in mathematics as a key arena for the production of young masculine identities, this article goes some way to addressing what is absent from sociological portrayals of young children, gender and mathematics education. Drawing on data derived from a top set mathematics group comprising five boys and one girl in a Grade 2 classroom, the article examines the ways in which a group's social dynamics interacted to define and regulate hegemonic masculinity. Boys in this study associate with mathematics, engage in oppositional and gendered discourses, claim power over girls and construct a version of masculinity which correlates with prestige and these have inequitable effects. I conclude by arguing that a focus on the early years using theoretical spaces offered by masculinity studies is really useful in understanding young boys' and girls' developing relationship with mathematics.