Citizen coproduction of public services can be a powerful force that influences employee work motivation. However, little research has been conducted to explore what kind of coproductive behaviors can motivate public employees in a positive direction. The two purposes of this article are to explore whether taxpayers would behave as tax collectors’ supervisors during their participation in the production and delivery processes of tax services, and whether such taxpayers’ supervisory behaviors would impact the job involvement of tax collectors. Drawing on the tax collectors of the tax collection agencies in Taipei, Taiwan, the authors first develop a coproductive-taxpayer-as-supervisor measure through focus group interviews and accepted psychometric testing procedures. The measure reveals that tax collectors would perceive taxpayers as their supervisors if taxpayers provide challenges and support to tax collectors. A mail survey further indicates that both taxpayers’ challenge and supportive behaviors are positively related to tax collectors’ job involvement.