South Africa is introducing Outcomes Based Education as an approach to teach the new Curriculum 2005 (C2005). The new curriculum contains exciting new aspects for the Natural Sciences. For example, in addition to learning traditional science subject matter, children are expected to develop also an understanding of the 'changing and contested nature of knowledge in the natural sciences', i.e. of the 'Nature of Science' (NOS). Since curriculum reform depends crucially on the teachers, it is important to find out whether their understandings of the NOS are in accordance with what they are required to teach. We explored existing understandings of the NOS among participants in several teacher professional development programmes through an internationally validated questionnaire backed up by semi-structured interviews. The questionnaire was expanded to elicit possible culturally and socially determined aspects of understandings of the NOS. We compare the teachers' understandings of the NOS with the understandings delineated in C2005. Teachers' understandings differ from those advocated in C2005 but match international research findings. Culturally determined views are found on the status of different knowledge systems. As a follow-up to this study, an exploration is under way into the impact of an in-service teacher-training programme directed at bringing teachers' understandings in line with the requirements of Curriculum 2005.