The purpose of this study was twofold: first, to examine the effectiveness of an inquiry-based laboratory course on preservice science teachers' (PSTs') science teaching efficacy beliefs. Second, this study also aimed to identify the ways in which efficacy change occurred in instructional strategies, classroom management, and student engagement for PSTs after participating in a semester-long inquiry-based laboratory course. A mixed-method approach was employed in this study. Data were collected through Teachers' Sense of Efficacy Scale (TSES) and semi-structured interviews. TSES was administered to 52 junior PSTs. Based on their initial levels of teaching efficacy (low, medium, high) determined by the TSES, three participants within each group were interviewed to obtain detailed insights regarding their science teaching efficacy. Quantitative data analysis suggested that PSTs' science teaching efficacy beliefs regarding student engagement, instructional strategies, and classroom management increased at the end of the course. Qualitative data analyses revealed that most PSTs expressed increased efficacy in sub-dimensions of teaching efficacy. Exceptional findings emerged delineating little or no efficacy change particularly in classroom management due to the discomfort with adapting to the inquiry-based laboratory method and lack of teaching practice in a real classroom environment. Results were discussed and suggestions for future research were provided.