As student-teacher-scientist partnerships become more widespread, there is a need for research to understand the roles assumed by scientists and teachers as they interact with students in general and in inquiry learning environments in particular. Although teacher roles during inquiry learning have been studied, there is a paucity of research about the roles that scientists assume in their interactions with students. Socio-cultural perspectives on learning emphasize social interaction as a means for students to make meaning of scientific ideas. Thus, this naturalistic study of classroom discourse aims to explore the ways scientists and teachers help high school students make meaning during authentic inquiry investigations. Conversational analysis is conducted of video recordings of discussions between students and teachers and students and scientists from two instances of a student-teacher-scientist partnership program. A social semiotic analytic framework is used to interpret the actions of scientists and teachers. The results indicate a range of common and distinct roles for scientists and teachers with respect to the conceptual, social, pedagogical, and epistemological aspects of meaning making. While scientists provided conceptual and epistemological support related to their scientific expertise, such as explaining scientific phenomena or aspects of the nature of science, teachers played a critical role in ensuring students' access to this knowledge. The results have implications for managing the division of labor between scientists and teachers in partnership programs.