Brand communities and corporate social responsibility have been touted for their ability to both generate significant equity for their brands and strong bonds among community members. However, the reciprocal capacity of these brands to mobilize their followers to engage in prosocial behavior, while very promising, has been largely ignored by researchers and practitioners alike. Given the untapped potential of product aficionados, who are said to possess considerable knowledge, expertise, and engagement, this lack of research and managerial guidance is somewhat perplexing. To address this shortcoming, we conducted a longitudinal investigation of two brand communities and one non-brand based product category community drawn from five years of data from a scientific crowdsourcing project aimed at curing medical diseases, such as Alzheimer's, Parkinson's, and AIDS. Employing a hazard modeling approach, we find that contrary to conventional wisdom, brand community participation reduces the likelihood of joining, while participation in a non-brand based product category community increases the likelihood of joining, a prosocial distributed computing project. Furthermore, willingness to help fellow community members plays an important role in the likelihood to join. These findings provide important insights for firms seeking to engage product category and brand communities in prosocial causes. (C) 2017 Direct Marketing Educational Foundation, Inc., dba Marketing EDGE. All rights reserved.