Mentors play a crucial role in preservice teachers' (PTs) professional development. Identity construction has not however been examined in the context of specific mentoring relationships, or as a process in motion. Drawing on the theory of the Dialogical Self, and its notion of dialogically constituted identities (I-positions), this case study explores PT identity construction and adaptive dynamics in two mentoring relationships. Identity narratives created during a five-week school placement are examined. Results show how, in a mentoring relationship, an internal I-position referencing a particular teaching identity is constructed in relation to an external I-position representing the PT's mentor. They demonstrate how a shift between teaching identities (internal I-positions) can be understood as a response to a perspective ‘voiced’ by an external position. Findings suggest that, in mentoring contexts, identity needs to be understood as constructed in interactions between self and other within a dynamically shifting relationship.