In this paper we propose the construct of discursive identity as a way to examine student discourse. We drew from the work of Gee (2001, Review of Research in Education, 25, 99-125) and Nasir and Saxe (2003, Educational Researcher, 32(5), 14-18) to consider the multiple contexts and developmental timescales of student discursive identity development. We argue that theories of scientific literacy need to consider the sociocultural contexts of language use in order to examine fully affiliation and alienation associated with appropriation of scientific discourse. As an illustrative case, we apply discursive identity to series of short exchanges in a fifth-grade classroom of African-American students. The discussion examines potential co-construction of student identity and scientific literacy.