This paper argues that an examination of expert-novice relationships in unfolding interaction should not proceed from the static and unidirectional view that knowledge and status are distributed as functions of a priori categories such as age, gender, and hierarchical rank. Although analysis of interactional sequences from the group meetings of a university physics team reveals the co-occurrence of professional status and expertise in some segments of the data, we show, through a conversation analytic approach, that the constitution of expert-novice in dynamic interaction is a much more complicated, shifting, moment-by-moment reconstruction of Self and Other, whether within a speaker's talk or between speakers. We demonstrate that the constitution of a participant as expert at any moment in ongoing interaction can also be a simultaneous constitution of some other participant (or participants) as less expert, and that these interactionally achieved identities are only candidate constitutions of Self and Other until some next interactional move either ratifies or rejects them in some way. This way of viewing expert-novice relations can help account not only for the bidirectionality postulated in those models of apprenticeship, socialization, and learning which are based on activity theory but also for change and innovation in communities of practice. The implication for research raised by this study is that the analysis of language use ought to go beyond the extrinsic social, cultural, and biological identities of speakers and recipients; it should include an analysis of how utterances constitute these identities and how utterances are organized despite these identities.