Teacher efficacy has become an important field of research especially in subjects teachers may find challenging, such as science. This study investigates the sources of teachers' efficacy for teaching science in primary schools in the context of authentic teaching situations with a view to better understanding sources of teachers' efficacy beliefs. Twenty-four teachers participated in focus group interviews to enable in-depth exploration of the sources of efficacy for teaching science. Data was analysed using a content analysis approach guided by a conceptual framework for efficacy in science teaching. Findings show efficacy to be influenced by mastery experiences, vicarious experiences, physiological and affective states as well as prior beliefs and experiences of science. Most notably, efficacy for science teaching was found to be enhanced through particular aspects of the teaching context such as opportunities for collaboration and successful participation in science teaching practice. Implications for teaching and teacher educators are discussed. (Contains 2 figures.)