Little management theory or empirical results exist to guide managers in maximizing employee efforts to successfully implement Environmental Management Systems (EMS). In response, this study examines the relationship among Human Resource (HR) factors and employee perceptions of environmental performance. Four hundred thirty-seven (437) employees were surveyed in an organization with a well-developed EMS program and ISO 14001 certification. Results suggest that management support for EMS, EMS training, employee empowerment, and EMS rewards are related to perceived environmental performance. Furthermore, EMS teamwork plays a mediating role between some of independent variables and perceived environmental performance. Finally, implications for managers are discussed.