This paper examines the relevance of differential language expertise in ordinary conversation between speakers of Japanese as a first and second language. Adopting a conversation analytic perspective, the study focuses on other-repair as one sequential environment in which the participants recurrently orient to their differential linguistic knowledge. Specifically, it will be shown that language expertise was made relevant (a) when one participant invited the other party's repair and (b) when the participants encountered a problem in achieving mutual understanding. On such occasions, the interlocutors oriented to the differences in their linguistic knowledge through their talk and other interactional conduct. The study thus provides evidence for differential language expertise as a participant category that emerges on occasion but bears no relevance for the participants during most of their talk.