The purpose of this mixed-methods research was to investigate changes in preservice elementary teachers' science teacher identities and self-efficacy beliefs as they participate in a field-based science methods course. A total of 121 preservice teachers participated, four of which were purposefully selected who held varied initial levels of science content preparedness and confidence to teach. Data sources included pre- and post-course administrations of the Science Teaching Efficacy Belief Instrument-B, an open-ended questionnaire, two semi-structured interviews with selected participants, written teaching reflections, classroom observations, and artifacts. Data analyses included a pre-post repeated measures analysis of variance (ANOVA) design, and the case study approach. The themes generated by using open and axial coding belonged to Bandura's sources of self-efficacy and Gee's identity framework. Results indicated statistically significant gains in participants' science self-efficacy beliefs. Qualitative analysis revealed that the nature of prior science experiences shaped participants' self-efficacy and identity uniquely. Findings are summarized under four major themes suggesting ways in which Bandura's sources of self-efficacy played role in shaping preservice teachers' identity. Emerging from this study was the close connection between the two constructs- self-efficacy and science teacher identity. The study includes implications for preservice teacher education programs and research.