In this study we present new insights into the relationships surrounding employees' feelings of psychological ownership (PO) at work and the resulting effects on contextual performance. Characterized by a feeling of possession, PO has been theorized to have positive influences on the attitudes and behaviors of employees. To explore PO's effects, we conducted a three-stage questionnaire survey of employees and their supervisors at a multinational pharmaceutical company in Taiwan. To help attenuate the errors associated with common method variance that trouble most studies using simultaneous data, a longitudinal approach was taken to investigate the relationships between person-organization fit, person-job fit, and PO. During the first stage of data collection, 330 questionnaires were sent to employees, and 168 (51%) valid questionnaires were returned. During the second stage, questionnaires were sent to the 168 employees who had responded to the first-stage survey. There were 134 (80%) valid questionnaires returned. Finally, during the third stage, questionnaires were sent to the supervisors of all 134 employees who had responded to the second-stage survey. There were 88 (66%) valid questionnaires returned. Findings reveal that person-job fit was significantly related to PO, and PO was significantly related to contextual performance. However, contrary to predictions, person-organization fit was positively, but not significantly, related to PO. We conclude by discussing the implications of these findings for both academics and practitioners and make suggestions for future research.