The purpose of this study was to examine the relationship between organizational socialization tactics, newcomers’ self-efficacy, proactive behaviors, and socialization outcomes. Based on a sample of 140 co-op university students who completed surveys at the end of their work term, the results indicated that newcomers’ self-efficacy and institutionalized socialization tactics were positively related to newcomer proactive behaviors. The results also indicated that newcomer proactive behaviors partially mediated the relationship between their self-efficacy and organizational socialization tactics with a number of proximal and distal socialization outcomes. Furthermore, feedback-seeking and information-seeking moderated the relationship between socialization tactics and several socialization outcomes. Institutionalized socialization tactics were more strongly related to socialization outcomes for newcomers who engaged in less feedback-seeking and information-seeking behavior. These results support an interactionist approach to organizational socialization in which newcomers’ self-efficacy, proactive behavior, and organizational socialization tactics all contribute to newcomers’ adjustment and socialization.