Online citizen science projects have the potential to engage thousands of participants with scientific research. A small number of projects such as Foldit use an online computer game format. Motivation to participate in Foldit was investigated in a group of 37 players using an online survey, semistructured interviews, and participant observation. Results suggest that contributing to scientific research and an interest in science were among the most important motivations for participation. Interaction with others within the community of participants and the intellectual challenge of the game were also key for the continuing involvement of this group of regular contributors.