Teachers' subject matter knowledge (SMK) is one factor contributing to teaching 'successfully', as this provides a basis from which pedagogical content knowledge develops. UK-based trainee science teachers teach all sciences to age 14 and often up to age 16. Trainees have specialist science knowledge in chemistry, physics, or biology from their degrees. Other sciences may not have been studied since school. Thus, trainee science teachers often teach 'outside specialism'. The extent to which teaching within and outside specialism influences successful teaching, ensuring learning objectives are achieved, was investigated. The sources seventy-one trainees use for preparing within and outside specialism science lessons for 11-14-year-olds and 14-16-year-olds and effects on teacher self-confidence of working in these two domains were probed by questionnaire and interview. All trainees responded to open and closed questions, and Likert-scale statements exploring preferences for teaching, self-confidence, handling subject-related questions within and outside specialism, and attitudes towards learning new SMK. A subgroup of 12 trainees participated in individual semi-structured interviews. The results are counter-intuitive: trainees teach more successful lessons outside their specialism, particularly in the early stages. This relates to using a richer range of SMK sources, including, crucially, advice from experienced colleagues. Within specialism, trainees report an inability to select appropriate knowledge and/or strategies and a sense of conflict in teaching inaccurate information. Some 'anxious' trainees rely heavily on extant materials for outside specialism teaching. 'Super-confident' trainees able to teach any science focus on selection of appropriate instructional strategies and realize early on the need to transform SMK. (Contains 2 notes and 4 tables.)