We studied virtual organizational learning in open source software (OSS) development projects. Specifically, our research focused on learning effects of OSS projects and the factors that affect the learning process. The number and percentage of resolved bugs and bug resolution time of 118 SourceForge.net OSS projects were used to measure the learning effects. Projects were characterized by project type, number and experience of developers, number of bugs, and bug resolution time. Our results provided evidence of virtual organizational learning in OSS development projects and support for several factors as determinants of performance. Team size was a significant predictor, with mid-sized project teams functioning best. Teams of three to seven developers exhibited the highest efficiency over time and teams of eight to 15 produced the lowest mean time for bug resolution. Increasing the percentage of bugs assigned to specific developers or boosting developer participation in other OSS projects also improved performance. Furthermore, project type introduced variability in project team performance.