We investigate fifth-grade students' identity work and science learning at the margins of a science classroom. By "margins" we refer to activities unrelated to formal classroom instructional content and practices, and which unfold across many settings and contexts, including the classroom itself, but also multi-party, social group gatherings during recess, field trips, and the home. Data were collected in ethnographic fashion, in both field notes and video records, and data analysis focused on the interactional details in which students' multiple identities were actively negotiated. Our descriptive analysis reveals some novel types of identities (e.g. collective and "occupational") that students enacted in daily social situations, which suggests that students' identity repertoire is more extensive and diverse than usually accounted for. In addition, students' identity work at the margins was most frequently tied to the dynamics and structure of the locally established, small-sized social groups in which students routinely clustered and thus reflected locally grounded identities (rather than normative categories of gender, race, and class identities). Finally, because students' activities shared continuities with biology content and practices, knowing and learning of biology were implicated in their identity work. We close by drawing implications of these findings for contemporary pedagogical approaches and teachers' instructional practices.