The impact of the non-native speaker's (NNS) language proficiency on their personal teaching performance has often been an issue in TESOL teacher education programmes. To explore this issue a study was conducted to investigate the link between language teachers' language proficiency and their teaching effectiveness. Classes taught by eight NNS teachers teaching the same or similar content were observed. Teaching effectiveness and teacher language proficiency were measured both through self-ratings and by independent raters. Teachers were compared in terms of such criteria as the quantity and quality of input provided, the amount and accuracy of metalanguage used, the extent and quality of feedback offered as well as classroom management skills. The results suggest that while language proficiency contributes to teaching effectiveness, other aspects of teaching are unrelated to a teacher's command of their second language (L2).