Drawing on trait activation theory this study examines the association of personality traits and task characteristics with job performance through the mediating role of organizational citizenship behavior. Consistent with this theory we also test the moderating influence of task characteristics on this mediation mechanism. Based on a sample of 648 employees of a German company and using structural equation modeling, we found that organizational citizenship behavior partially mediates the relationships between personality traits as well as task characteristics and job performance. The study identifies the specific pathways through which personality traits (conscientiousness and sportsmanship) and task characteristics (conscientiousness and civic virtue) are related to job performance. The proposed moderating influence of task characteristics on the mediation mechanism is not supported. The findings suggest that specific organizational citizenship behavior dimensions function as a mechanism through which personality traits as well as task characteristics are associated with job performance.