Science performance assessments (SPAs) are designed to elicit a wider range of scientific knowledge and abilities than ordinarily measured by more traditional paper-and-pencil tests. To engage in SPAs and thus demonstrate abilities such as scientific inquiry, students must interact with various participants and communicate in a variety of ways. Using an analytical framework that draws on functional and interactional views of language, we analyzed video footage of one SPA, "The World Down Under," enacted in three fifth-grade classrooms. The video analysis revealed a wide variety of language demands that students encountered during the SPA, including a range of participant structures (whole class, small group, and individual), communicative modes (interpreting language, presenting language, and engaging interpersonally), and 18 different texts. Moreover, the language demands differed by classroom. A closer look at two video episodes illustrated how this more expansive communicative context afforded two English Learners (ELs) with opportunities to navigate the language demands of the assessment. However, the same language demands could present challenges for ELs with lower levels of English proficiency. The findings contribute to science education by bringing attention to the nature and role of language in SPAs, which has only begun to be described in the literature.