Previous research on English Medium Instruction classrooms has almost exclusively documented the interaction between a teacher and students where the teacher is able to understand and use the first language (L1) of the students. We present data obtained from science classrooms in English-medium-instruction (EMI) high-school programs in China where the teacher had almost no L1 proficiency and therefore spoke only English in class. The beliefs of both teachers and students were elicited with regard to L2-only English use in classroom interaction through questionnaire and post-lesson interviews with references to lesson observations. Two hundred and thirty-one questionnaires were collected, together with 60 interviews with students and teachers following 30 lesson observations. Our findings suggest that teachers divided into three categories of beliefs: virtual position, maximal position, and optimal position (already identified in previous literature), but that students tended to prefer a classroom environment in which some L1 was permitted, or even encouraged, in order to facilitate their understanding of complex science constructs. With increasing teacher mobility and a growing demand of EMI education worldwide, pedagogical suggestions are made to EMI programs with monolingual teachers.