We examine motivations for prosocial behavior using new data on volunteer firefighters that contain a dictator-game based measure of altruism, surveyed measures of other behavioral factors, and call records that provide an objective measure of time spent volunteering. Controlling for a variety of other explanations, we find that altruism and reputational concerns are positively associated with the decision to volunteer. Moreover, by utilizing variation in the presence and level of small stipends paid to the firefighters, we find that the positive effect of monetary incentives declines with reputational concerns, supporting a prediction that extrinsic incentives can crowd out prosocial behavior.