Recent education reform efforts have included an increasing push for school science to better mirror authentic scientific endeavor, including a focus on science practices. However, despite expectations that all students engage in these language-rich practices, little prior research has focused on how such opportunities will be created for English-learning students. This case study uses the conceptual framework of communities of practice to investigate the relationship between English-learning students' argumentation and their middle school sheltered English immersion (SEI) science classroom community. Considering various aspects of this conceptual framework--including the role of legitimate peripheral participation, as well as the degree to which community members' goals and expectations around the practice of interest align--allowed us to identify classroom characteristics that both hindered and facilitated students' opportunities to engage in argumentation. First, the classroom community, and consequently the presence and quality of argumentative discourse, was influenced by student movement in and out of this classroom, as their English proficiencies improved. The constantly changing class roster made it difficult for newer members to watch, learn, and engage in argumentation with more knowledgeable peers. Furthermore, certain elements of the SEI approach, namely its deductive nature, conflicted with the type of instruction necessary to encourage language use for sensemaking. However, this instructional context also offered English-learning students with affordances they may not find in other educational settings. Specifically, we found that when students worked in smaller group structures, such as pairs, and they utilized both their native and second language as a linguistic resource for engaging in science discourse, their engagement in argumentation was promoted.