The purpose of this study was to examine the influence of Albert Bandura's four hypothesized sources of self-efficacy on students' writing self-efficacy beliefs (N = 1256) and to explore how these sources differ as a function of gender and academic level (elementary, middle, high). Consistent with the tenets of self-efficacy theory, each of the sources significantly correlated with writing self-efficacy and with each other. As hypothesized, students' perceived mastery experience accounted for the greatest proportion of the variance in writing self-efficacy. This was the case for girls and for boys, as well as for students in elementary school, middle school, and high school. Social persuasions and anxiety also predicted self-efficacy, albeit modestly. Vicarious experience did not predict writing self-efficacy. Girls reported greater mastery experience, vicarious experience, and social persuasions, as well as lower writing anxiety. Girls also reported stronger writing self-efficacy and were rated better writers by their teachers. Elementary school students reported stronger mastery experience, vicarious experience, and social persuasions than did either middle school or high school students. Elementary school students also reported stronger self-efficacy. Findings support and refine the theoretical tenets of Bandura's social cognitive theory. (Contains 3 tables.)