This quasi-experimental study tested the potency of different sources of self-efficacy beliefs. Respondents were primary teachers (N = 93) in 9 schools who completed surveys of their self-efficacy beliefs and level of implementation of a new teaching strategy for beginning readers before and after participating in 1 of 4 formats of professional development presenting the same teaching strategy with increasing levels of efficacy-relevant input. Results indicated that the professional development format that supported mastery experiences through follow-up coaching had the strongest effect on self-efficacy beliefs for reading instruction as well as for implementation of the new strategy. A substantial proportion of the teachers who participated in formats that included a demonstration with local students and a planning and practice session, but no follow-up coaching, experienced a decrease in their self-efficacy for reading instruction. (Contains 4 notes and 1 table.)