For language teachers who are concerned about referring to their own and students' identities other than in the roles of "teacher" and "student" in the classroom, this conversation analytic study aims to give insights into the use of identity. Detailed analysis of the data of English for a Specific Purpose (ESP) classrooms indicates that contrasting the teacher's and students' non-default situated identities, such as "senpai" ("senior" in English) with "kohai" ("junior" in English) and sociologist with scientist, is a way for the language teacher to perform the role of "teacher" effectively in ESP classrooms: the practice constructs an epistemic gradient among the teacher and the students and makes some actions accountable by the participants, who is ascribed a superior epistemic status with an identity. The study concludes with a discussion of the contribution the use of identity can make to ESP/LSP (language for specific purposes) and suggestions for ESP/LSP course development.