While the use of ethnographic and qualitative research methods has spread across many fields of educational inquiry, until recently this has had little impact on research in science education. However, there is currently a methodological shift taking place in this area, prompted by the rise of a sociocultural perspective on cognition, which sees knowledge as tied to action, and treats expertise as being defined by relevant communities of practice. There is a close affinity between this perspective and ethnography. In this paper, I discuss a recent ethnographic investigation of primary science expertise guided by this perspective: an in-depth study of the perspective and practice of a single teacher. I focus on some of the problems involved in this study, arising from the divergent orientations of the researcher and the practitioner in the context of their collaboration. These difficulties threatened the completion of the study, but in the end were negotiated successfully. However, they raise important questions about the relationship between ethnographer and informant in the context of ethnographic research.