Community service is widely regarded as a fundamental experience in preparation for good citizenship, but it remains unclear whether common variants of service are consequential for civic outcomes. This study examines changes in the relative importance assigned to prosocial and egoistic values associated with service through different types of organizations, service prompted by external contingencies, and service that spans a narrow or wide frame of time. Data were drawn from the survey responses of 16,749 secondary school students (50 percent female, 28 percent ethnic minority, modal age = 15) who participated in the National Educational Longitudinal Study during their sophomore year in 1990 and completed a follow-up survey during their senior year in 1992. Results from a propensity score analysis indicated that service through humanitarian organizations but not other types of organizations was positively associated with the adoption of a prosocial value orientation. Service prompted by an institutional mandate or social pressure was negatively associated with prosociality relative to service characterized as strictly voluntary, although all students except those with an initially egoistic value orientation benefited from mandatory service relative to no service. While short-term service during the sophomore year had no enduring effect, long-term service predicted gains in prosociality above and beyond the effect of concurrent service. The findings suggest that the relationship between community service and prosocial value development can be optimized to the extent that service through a humanitarian organization is prompted by autonomy-supportive conditions over the course of 2 years.