This paper describes the effects of science teacher subject-matter knowledge on classroom discourse at the level of individual utterances. It details one of three parallel analyses conducted in a year-long study of language in the classrooms of four new biology teachers. The conceptual framework of the study predicted that when teaching unfamiliar subject matter, teachers would use a variety of discourse strategies to constrain student talk to a narrowly circumscribed topic domain. The paper includes the results of an utterance-by-utterance analysis of teacher and student talk in a 30-lesson sample of science instruction. Data are broken down by classroom activity for a number of measures, including duration of utterances, domination of the speaking floor by the teacher, frequency of teacher questioning, cognitive level of teacher questions, and student verbal participation. When teaching unfamiliar topics, the four teachers in this study tended to talk more frequently and for longer periods of time, ask questions frequently, and rely heavily on low cognitive-level questions. The rate of student questions to the teacher was found to vary with the classroom activity. In common classroom communicative settings, student questions were less common when the teacher was teaching unfamiliar subject matter. The implications of these findings include a suggestion that teacher knowledge may be an important unconsidered variable in research on the cognitive level of questions and teacher wait-time. (Author/CW)