Partnerships between scientists and teachers are an important focus of the current reform in science education. This study examined the roles and the dynamics of interactions in an NSF-funded Graduate Teaching Fellows in K-12 Education (GK-12) project. Data sources included interviews with teachers, fellows, and students at eight K-12 schools. Data were analyzed for emergent categories recognized by teachers and fellows alike. Roles played by the fellows were those of science/ math expert, scientist/mathematician role model, source of material resources, source of curricular enrichment, and teaching partner. Teacher roles were perceived as liaison between fellow and schools, teaching partner, teaching mentor, and science/math learner. Although the project underdefined the roles of the teachers, teachers showed noteworthy consistency in perception of their roles. The roles of the fellows differed from those of the project description and evolved idiosyncratically within each teacher-fellow partnership. Because a consistent understanding between project directors and participants regarding roles is critical to the success of a project, the authors suggest that further research into roles that emerge within part­nerships is vital for the future development of K-12 projects.