Based on linguistic ethnographic research in a majority Latino/a, lower-income high school in southern Arizona, this article illuminates the fluid, contingent, and contested nature of expertise in a science classroom. Over one academic year, a teacher's talk about discourse practices and behaviors associated with scientists set the stage for individual interactions in which she socialized students to make knowledge claims like scientists. Thus, interdiscursive relations, or connections between similar kinds of speech events, were crucial to the emergence of expertise in particular interactions. At the same time, students displayed cultural and linguistic expertise in science-related interactions, socializing the teacher into understandings and discourse practices associated with the students' cultural identities. Analysis of video-recordings of classroom interaction highlights the central role of participants' epistemic practice, or their management of knowledge claims, in these processes of socialization. The findings are relevant to linguistic anthropologists as well as science educators who seek to promote equity for students from traditionally marginalized groups.