Recognition of the critical role of indigenous knowledge systems (IKS) in science in the new South African national curriculum for schools affirms the importance of IKS, particularly in the natural, physical and life sciences, as they are included in its policy. This paper explores the question of how teachers implement IKS in their science teaching when provided with the opportunity to do so. The study draws on a theoretical methodological framework developed for researching educational possibilities that take account of the current situation, the imagined situation for introducing IKS in science, and an arranged situation created to study "what is not yet but could be" in science classrooms. Drawing on a range of data sources that began with a survey of teachers taking a postgraduate science education module, followed by IKS-related task analysis and interviews, three teachers from diverse science classrooms were selected for observations and further interviews. The analysis revealed three approaches to engaging IKS, which may be characterised as: an incorporationist approach that brings selected indigenous knowledge into science by seeking how "best IKS fits into science"; a separatist approach that holds IKS "side-by-side" with scientific knowledge; and an integrationist approach that makes "connections" between IKS and science.