This study explores the opportunity provided to science teachers to incorporate teaching approaches compatible with constructivist views of learning into competency based teaching. In doing so, the study aims to investigate the extent to which the educational philosophies these different curriculum frameworks represent are compatible. The results indicate that teachers' perceptions of the goals of constructivist competency based science programs and their actions within the classroom are influenced by their established pedagogical beliefs, their concerns about how the programs will affect them directly within the classroom, their beliefs about science teaching and learning and the nature of science and their own personal objectives for teaching. For many teachers, the constructivist programs created contradictions between their own beliefs about science teaching and science knowledge and what this program expects of them as teachers. Where teachers came to the programs with constructivist views of learning and teaching, they were more likely to adopt teaching approaches, in particular problem based approaches, which were compatible with the constructivist intentions of the programs. On the other hand, teachers who brought with them more traditional beliefs about science and about learning and teaching, were constrained in how they could respond to the programs. They translated the constructivist intentions of the various competencies, in particular problem solving and organising and communicating scientific ideas, so that, to a large extent, they were able to continue doing in the classroom what they previously had been doing. Teachers perceived that the competency based programs required of them a role very different from that required in the traditional programs. In particular, teachers considered that a fundamental aspect of the competency based programs was a shift in responsibility for learning from the teacher to the student. However, again, the ideas of learner control and student responsibility for learning were interpreted by teachers in ways which enabled them to continue in established roles within their classrooms with which they were comfortable. Without complementary sets of beliefs about the nature of science and science teaching it would appear that teachers will have great difficulty understanding the intentions of the new programs. And, that even where they do have these beliefs, they may still be constrained by the lack of appropriate images of what constructivist teaching and learning environments might look like.