As the de facto language of science, English proficiency often serves as a gatekeeper for aspiring medical students in Puerto Rico because they encounter English-language instruction, textbooks, and high-stakes standardized tests throughout their studies. Employing ethnographic research methods, this case study documents the language practices in an Evolution course at a bilingual Puerto Rican university, which was cotaught by two non-Puerto Rican professors, neither of whom use English as their native language. Using interviews, a focus group, and video-recorded data, this study analyzes the language practices and pedagogies of both professors as they teach the same course and attempt to make the content comprehensible for their students. Our findings document the nonpurist linguistic behavior of the professors and how they built on their students' linguistic repertoires. Findings also point to how they adopted a nonprescriptive pedagogy that challenges pervasive monolingual ideologies in STEM.