This paper focuses on the constitution of the professional identity of English teachers from Brazilian pubic schools, based on their own written evaluations of classes. From the perspective of the Socio-discursive Interactionism (Bronckart, 1997), the study discusses how the development of argumentation can trigger the constitution of critical citizenship. The object of the analyses were written evaluations collected during the module "O papel do multiplicador" of the program "Reflexão sobre a ação: o professor de inglês aprendendo e ensinando", which is offered by Sociedade de Cultura Inglesa and PUC-SP to public school teachers. Initial results show that argumentation allows the development of teachers' self consciousness about their own actions in a historical perspective that leads to a better understanding of their own identities (Moita Lopes, 1998).