The purpose of this study was to empirically investigate the effect of mentor teachers' leadership practices on student interns' efficacious beliefs in Professional Development Schools (PDSs) during the internship practicum. The study employed the Leadership Practices Inventory (Kouzes & Posner, 2003) to measure the transformational leadership practices of mentor teachers. Tschannen-Moran and Hoy's Teacher Sense of Efficacy Scale (2001) was utilized to evaluate student interns' efficacious beliefs. Data were collected via survey questionnaires. The sample included 154 student interns from a mid-sized Mid-Atlantic university who were participating in their internship practicum within 33 Professional Development Schools during the spring of 2009. This study found that teachers' efficacy beliefs in student engagement, instructional strategies, and classroom management are positively related to the total scale of the leadership practice of mentor teachers and each of the five subscales (modeling the way, inspiring a shared vision, challenging the process, enabling others to act, and encouraging the heart), except for the insignificant relationship between modeling the way and teacher's efficacy of student engagement. (Contains 2 tables.)