Guided by sociological perspectives that view (young) children as agents who are shaped by and shape the structures of social systems in which they live, we studied forms that agency of young children from historically marginalized groups may take in science class when offered opportunities to engage in science in a variety of ways, how curricular and instructional structures facilitate or hinder these children's agency, and how their agency influence these structures. Using a case study approach, we analyzed performance and narrative data related to Carlos, a Latino 3rd grader, to understand the dialectical relationship between Carlos's agency and structural elements of his year-long science class as a sociocultural system populated with people, curricular and instructional structures, research structures, artifacts, and tools. Carlos performed and narrated his self in relation to science differently across the various structures in which he found himself--as a knowledge broker, a (self-appointed) authority, an emergent author, and a self-identified scientist. His agency was linked to the roles he performed within the structures, roles he created at the intersection of the rules and resources that governed the structures in which he was immersed, and of his construction and interpretation of them. Learning from Carlos's case, we discuss implications for teachers of science to recognize the many ways in which students make meaning and plan for them, and to craft ideologies that embrace and nurture student agency in relation to structures--an essential understanding that should inform teaching and teacher education for justice and equity.