Self-efficacy beliefs that relate to teachers' motivation and performance have been an important area of concern for preservice teacher education. Research suggests high-quality science coursework has the potential to shape preservice teachers' science self-efficacy beliefs. However, there are few studies examining the relationship between science self-efficacy beliefs and science content knowledge. The purpose of this mixed methods study is to investigate changes in preservice teachers' science self-efficacy beliefs and science content knowledge and the relationship between the two variables as they co-evolve in a specialized science content course. Results from pre- and post-course administrations of the Science Teaching Efficacy Belief Instrument-B (Bleicher, 2004) and a physical science concept test along with semi-structured interviews, classroom observations and artifacts served as data sources for the study. The 18 participants belonged to three groups representing low, medium and high initial levels of self-efficacy beliefs. A repeated measures multivariate analysis of variance design was used to test the significance of differences between the pre- and post-surveys across time. Results indicated statistically significant gains in participants' science self-efficacy beliefs and science conceptual understandings. Additionally, a positive moderate relationship between gains in science conceptual understandings and gains in personal science teaching efficacy beliefs was found. Qualitative analysis of the participants' responses indicated positive shifts in their science teacher self-image and they credited their experiences in the course as sources of new levels of confidence to teach science. The study includes implications for preservice teacher education programs, science teacher education, and research.