One chronic challenge facing professional development providers is the need to convene and support groups of role-alike K-12 teachers who require similar kinds of discipline-and grade-level-specific training. Online courses have become an increasingly common way to meet this challenge. This article reports on a randomized control trial that tested the impact of one such course on teachers of high-school biology and their students. The course, Teaching High School Biology, sought to improve teachers' content knowledge, pedagogical knowledge, and knowledge of how to integrate digital media resources into their teaching. The course was developed by WGBH's Teachers' Domain and delivered by PBS TeacherLine. The study finds that teacher participation in the course had a significant impact on teacher knowledge but did not improve student outcomes. The authors note that the course sought to help teachers learn about inquiry-oriented teaching methods that are difficult to implement in ways that have a significant impact on student learning. Future research would benefit by attending to whether and how an online professional development course aligns with local curricula and policy priorities that shape participants' instruction.