School science students can benefit greatly from participation in student-directed, open-ended scientific inquiry projects. For various possible reasons, however, students tend not to be engaged in such inquiries. Among factors that may limit their opportunities to engage in open-ended inquiries of their design are teachers' conceptions about science. To explore possible relationships between teachers' conceptions about science and the types of inquiry activities in which they engage students, instrumental case studies of five secondary science teachers were developed, using field notes, repertory grids, samples of lesson plans and student activities, and semistructured interviews. Based on constructivist grounded theory analysis, participating teachers' tendencies to promote student-directed, open-ended scientific inquiry projects seemed to correspond with positions about the nature of science to which they indicated adherence. A tendency to encourage and enable students to carry out student-directed, open-ended scientific inquiry projects appeared to be associated with adherence to social constructivist views about science. Teachers who opposed social constructivist views tended to prefer tight control of student knowledge building procedures and conclusions. We suggest that these results can be explained with reference to human psychological factors, including those associated with teachers' self-esteem and their relationships with knowledge-building processes in the discipline of their teaching.