A study examined the language use of two teachers, one a native, bilingual Cantonese-English speaker and the other a monolingual English-speaker, as they alternated teaching assignments between two first grade classes in a Chinese-English bilingual education program. Teacher-student interaction and the variation in teacher and student language use were observed and recorded. These data were coded using a modified system of conversational-acts (C-acts). The language was analyzed according to the utterances' grammatical structure, illocutionary properties, general semantic or propositional content, and for frequency and proportional usage. Results showed great consistency in the bilingual teacher's language use across groups, while the monolingual teacher's patterns of C-act use across language proficiency groups were quite different. Instructional organization and the use of the students' native language also varied between teachers relating to the different groups. The findings show that a monolingual teacher cannot act as effectively as the bilingual, for the monolingual is not familiar with language patterns that may cause confusion for second language learners. (MSE)