This investigation was guided by the following research questions: (1) how do teachers' understandings of the nature of science influence classroom practice?; and (2) what factors facilitate or impede the influence of teachers' understandings on classroom practice? An in-depth, year long assessment of the classroom practices and goals of five biology teachers (three males, two females), with a wide range of experience was conducted. A combination of semi-structured interviews, open-ended questionnaires, classroom observations, collected lesson plans and instructional materials, periodic informal interviews/discussions, and student interviews are used to investigate the relationship between teachers' conceptions of the nature of science and classroom practices and to elucidate those factors that impede or enhance the relationship. The sample teachers all possessed views consistent with those advocated by current reforms in science education. The teachers' conceptual frameworks of biology and pedagogy were assessed and it was found that consistent with previous research, the teachers' conceptions of the nature of science did not necessarily influence classroom practice. Implications for science teacher education and reform implementation are discussed. Contains 20 references. (LZ)