The purpose of this dissertation is to investigate the professional identity construction of a student becoming a teacher in the context of a private language school in Nilópolis, Rio de Janeiro. I study the development of this student from the moment he showed an interest in becoming a teacher until he joined the teaching staff of the same school, passing through his choice of studying at the institution`s Teachers Training Course and, later on, his entrance in a university Letters course. I characterize this research as a longitudinal case study (Lüdke e André, 2001), inspired by the principles of Exploratory Practice (Allwright, 2000; Moraes Bezerra, 2003). I analyze data from an advanced English class, in which the 24-year-old student expressed his interest in becoming a teacher; and data from 8 interviews, conducted during one year and a half, with the purpose of reflecting about his professional decisions and growth. In theoretical terms, I view identity as socially constructed (Moita Lopes, 2002; Dutra, 2003; Kleiman, 2003). I align myself with the socio-construcionist perspective on learning (Vygotsky, 1994; Bakhtin, 1981 e 1992; Mercer, 1994; Edwards e Mercer, 1994) and also consider discourse in its social dimension (Cook, 2000). I problematize identity construction issues in the process of the specific learner becoming a teacher. I highlight his initial belief in the concept of model and inspiration and his critical reflection about the importance of the university Letters course in teacher professional development.