In recent years much emphasis has been placed, both by researchers and by policy-makers, on the role that subject knowledge plays in the classroom practice of primary teachers. Within UK research on primary science education, this emphasis is often linked with constructivist ideas about effective teaching. In this article, I explore the implications of applying a rather different approach, based on sociocultural theories of cognition and learning. These stress the situated nature of knowledge and the complex interdependence of learning and action. Above all, these perspectives treat expertise as defined in action by relevant communities of practice. Thus, in this article, I draw upon data from an in-depth qualitative case study of one primary science teacher who is recognized in her local environment, and more widely, as an expert practitioner. I examine her views about subject knowledge, and her beliefs about the learning and teaching of science. I also investigate her practice. One outcome of this study is the conclusion that teacher expertise is "eclectic" in character, drawing on a variety of pedagogical strategies and theories of learning in dealing with the contingent situations faced in the classroom. I conclude by suggesting that this aspect of primary science practice is particularly important today, given that currently influential views configure teaching in terms of abstract standards concerned with level of subject and pedagogical knowledge.