We examine market structure and optimal commodity taxation in a world in which firms produce differentiated products and can exert some degree of market power. Building on Kay and Keen (1983), we model two forms of product technologies, two forms of market entry structures, and two forms of pricing. This yields eight models providing a richer analysis of the role of taxes as regulatory tools than could be provided in Kay and Keen’s analysis. In the presence of price discrimination, tax policy loses much of its effectiveness at serving as a substitute for direct regulation. Moreover, in cases where taxes can influence market structure, subsides rather than taxes may be required to achieve optimum market structure. Our results should remove the presumption that has developed over the past twenty years that the ad valorem tax rate should be positive to discourage excess entry in imperfectly competitive markets.