Web-based learning has been proposed as a convenient way to provide professional development experiences. Despite quantitative evidence that online instruction is equivalent to traditional methods (Russell, 2001), the efficiency of this approach has not been extensively studied among teachers. This case report describes learning in an online biology course designed to help teachers prepare for science certification exams. A mixed methodology approach was utilized to analyze the manner in which course participants learned and how the online environment influenced this process. Concept maps scored by two different methods and objective pre- and postcourse examinations were contrasted as representations of assimilated knowledge, while unstructured interviews reflected participants' perceptions of their experiences. Findings indicate that participants experienced gains in declarative knowledge, but little improvement with respect to more complex levels of understanding. Qualitative examination of concept maps demonstrated gaps in participants' understandings of key course ideas. Engagement in the use of online resources varied according to participants' attitudes toward online learning. Subjects also reported a lack of motivation to fully engage in the course due to busy schedules, lack of extrinsic rewards, and the absence of personal accountability.