Following the implementation of the new outcomes-based Curriculum 2005, teacher competence in teaching reform-based science in large classes is one of the challenges in the continuing reform of South Africa's education system. Most school teachers have little experience, meagre training and are operating in large and poorly resourced science classrooms. As part of the understanding of the teaching and learning that takes place in such contexts, this study focuses on science teachers' perspectives on their current classroom practices. Data collected through surveys and questionnaires to participating science teachers revealed their perspectives of the reality of teaching large under-resourced science classes and the typical patterns of classroom teaching. The essential components of their instructional functions are identified together with the implications these have for teacher education.