The purpose of this study was to explore how science teachers' epistemological beliefs and teaching goals are related to their use of lab activities. Research questions include (a) What are the teachers' epistemological beliefs pertaining to lab activities? (b) Why do the science teachers use lab activities? (c) How are the teachers' epistemological beliefs and instructional goals related to teaching actions? Two major aspects of epistemologies guided this study: ontological aspect (certainty/diversity of truth) and relational aspect (relationship between the knower and the known). The ontological aspect addresses whether one views knowledge as one certain truth or as tentative multiple truths. The relational aspect addresses whether one views him/herself as a receiver of prescribed knowledge separating self from knowledge construction or as an active meaning maker connecting self to the knowledge construction processes. More sophisticated epistemological beliefs include the acknowledgement of multiple interpretations of the same phenomena and active role of the knower in knowledge construction. Three experienced secondary science teachers were interviewed and observed throughout an academic course. The findings illustrate that a teacher's naïve epistemological beliefs are clearly reflected in the teacher's teaching practices. However, a teacher's sophisticated epistemological beliefs are not always clearly connected to the practice. This seems to be related to the necessary negotiation among their epistemological beliefs, teaching contexts, and instructional goals. Ontological and relational beliefs seem to be connected to different facets of teaching practices. Findings indicate that various syntheses of different aspects of epistemological beliefs and instructional goals are linked to teachers' diverse ways of using lab activities. Implications for research and teacher education are discussed. © 2004 Wiley Periodicals, Inc. Sci Ed89:140–165, 2005