Focusing on teaching situations with varying degrees of teacher knowledge, an interpretive study examined the effects of science teacher subject-matter knowledge on classroom questioning and other forms of discourse. Four biology and life science teachers participated in the first year of two year-long studies (the second is in progress). All of the teachers were interns or student teachers in a fifth-year teacher education program, and taught at the 9-12 grade level in San Francisco Bay area public high schools. The relative subject-matter knowledge of the teachers on 15 biological topics was determined using quantitative and qualitative discourse analysis. Results revealed that when teachers did not have a strong understanding of the topic they were teaching, they were likely to use frequent low cognitive-level questions to control classroom conversation, and student participation was minimal. In high teacher-knowledge classes, teachers asked fewer questions and student participation and questioning was high. (Figures of the study's conceptual framework, the middle strand of the conceptual framework (speech activities level), the rate of low cognitive-level teacher questioning, the frequency of student speech acts, student questioning rate, ratio of teacher to student questions, and class time spent on other activities before class discussion are included, and 19 references are appended.) (MM)