Due to the central role of textbooks in the teaching and learning of science, it is imperative that textbooks provide correct content and instructional support. We investigated how 16 South African Grade 7 natural science educators selected their textbooks and how they evaluated these textbooks. The results were analysed according to the constructivist paradigm, as implemented in the Revised National Curriculum Statement, as well as selection criteria found in the literature. The selection criteria that the educators listed in interviews were isolated ideas that were not embedded in a constructivist framework. The educators also lacked the necessary scientific and pedagogical content-knowledge to effectively evaluate the textbooks. Recommendations are made for improvements in educator training, departmental guidance, and textbook writing. Experience in textbook evaluation during their training courses could boost educators' competence in selecting the books and detecting and compensating for their deficiencies. This would contribute to filling South African educators' need of hands-on examples that can be applied immediately in their classrooms.