While the literature is replete with studies examining teacher knowledge and pedagogical content knowledge (PCK), few studies have investigated how science teacher orientations (STOs) shape classroom instruction. Therefore, this research explores the interplay between a STOs and the topic specificity of PCK across two science topics within a grade 9 earth science course. Through interviews and observations of one teacher's classroom across two sequentially taught, this research contests the notion that teachers hold a single way of conceptualising science teaching and learning. In this, we consider if multiple ontologies can provide potential explanatory power for characterising instructional enactments. In earlier work with the teacher in this study, using generic interview prompts and general discussions about science teaching and learning, we accepted the existence of a unitary STO and its promise of consistent reformed instruction in the classroom. However, upon close examination of instruction focused on different science topics, evidence was found to demonstrate the explanatory power of multiple ontologies for shaping characteristically different epistemological constructions across science topics. This research points to the need for care in generalising about teacher practice, as it reveals that a teacher's practice, and orientation, can vary, dependent on the context and science topics taught.
