This paper describes the progress of an experienced primary school teacher participating in a cooperative action research extending over a period of two years. The focus was on how she changed from confronting STS perspectives as unfamiliar ideas initially to gradually accepting them and finally feeling comfortable enough to put the theries into her classroom teaching practice. The predicaments she was in and the efforts she made in dealing with them are described. Through this case study, the researchers were able to reflect on the implications of constructivism and action research methods in the professional development of teachers and changes in their teaching practice.