A model of the relations among taxpayers' opportunity, social norms, ethical beliefs, and tax compliance is proposed and tested using structural equation modeling. High opportunity taxpayers, who may personally benefit from evasion, judged evasion as less unethical than low opportunity taxpayers. High and low opportunity taxpayers judged social norms similarly. Further, ethical beliefs partially (fully) mediate the relation between opportunity (social norms) and underreporting. Implications from our study to tax compliance researchers and policy makers are discussed.