The purpose of this mixed methods research was to investigate the development of 55 preservice elementary teachers' science teaching self-efficacy beliefs through analysis of their reflective practices in a science method course. This year-long study was conducted at two public universities located in the USA and Canada. Within the theoretical frameworks of science teaching self-efficacy and reflective practice, we examined how and in what ways preservice teachers' reflections on their past science experiences and current science teaching practices contributed to their self-efficacy beliefs. Data were collected from pre- and post-course administrations of the Science Teaching Efficacy Belief Instrument-B (STEBI-B), written science autobiographies, written reflection papers, classroom observations, and artifacts. A repeated-measures analysis of variance (ANOVA) was used to test the significance of the differences between the pre- and post-course survey scores. The results indicated statistically significant gains in participants' science teaching self-efficacy beliefs. The qualitative analysis revealed that preservice teachers' initial self-efficacy beliefs evolved through years of formal and informal interactions with science. New levels of self-efficacy were reached, as evident from participants' self-reflection on their field teaching. The findings indicate the importance of field experiences and reflective practices for preservice elementary teachers' preparation and science teaching self-efficacy. The study offers implications for preservice teacher education programs, science teacher education, and research.