Multilingual and multicultural societies in Europe pose new challenges for schools and have led to the expansion of the Content and Language Integrated Learning (CLIL) approach to foreign language teaching. English teachers in Catalan primary schools frequently face the challenge of integrating science teaching with foreign language teaching. This paper analyses how an English primary teacher's performance as a science-CLIL teacher evolves by means of coteaching with science teachers, and relates changes in her performance to the development of a science-CLIL teacher professional identity. An analytical framework has been constructed that includes: (1) a science-CLIL teacher professional identity model, and (2) a model that characterises collaborative practice in a cotaught science-CLIL classroom. The results indicate that coteaching is a powerful strategy for promoting discursive changes that can be considered evidence of the English teacher's science-CLIL teacher professional identity development through: (a) challenging her English teacher sub-identity to embrace multilingualism in the classroom; (b) developing a science teacher sub-identity that allows her to participate successfully as a science teacher and therefore to share leadership when teaching science, and (c) constructing a CLIL-teacher sub-identity that is more capable of balancing English teaching and science teaching.