A problem-based learning framework, which focused on developing preservice teachers' knowledge base for teaching and pedagogical reasoning ability, was developed for an undergraduate preservice science teacher education unit. Only three knowledge base components were considered in the design of the PBL problem scenario, namely science content knowledge, curriculum knowledge, and knowledge of the learner. The pedagogical reasoning model used in this study was based on six aspects: comprehension, transformation, instruction, evaluation, reflection, and new comprehension. The problem was designed so that preservice teachers were required to develop and apply their knowledge, through the use of a pedagogical reasoning process, to a typical primary classroom teaching situation. To ensure all six components of the pedagogical reasoning process could be considered, a peer teaching activity related to the problem was included as part of the unit. The PBL approach was trialed in a second-year preservice science education subject, which was part of a 3-year bachelor of teaching program. Participants worked in small groups (three or four) on the problem over a 6-week period. A case study approach was used to evaluate the knowledge base and pedagogical reasoning components considered by two randomly selected participants in this study. Each preservice teacher considered and developed their knowledge base and pedagogical reasoning in areas that were relevant to their current understanding of primary teaching and student learning.