This paper investigates the code-switching behaviour of two native speaker teachers teaching their mother tongue--French and Korean, respectively--to predominantly English monolingual students in New Zealand secondary schools. A close analysis of these teachers' classroom discourse and their perceptions about classroom language use reveals a range of factors which, in spite of their proficiency, constrain their use of the target language (TL) for teaching purposes. While the individual teachers' attitude toward TL use appeared to be the main determinant of their differing degrees of TL use for instruction, there were also institutional and societal factors which influenced the way they constructed their bilingual identities and manifested in different patterns of language choice. These factors included the status of the TL within the educational system, and the pervasive presence of English as the convenient vehicle for message conveyance in the wider context. Implications are drawn for teacher training and language policy.