AbstractUsing aCournot oligopoly model with an endogenous number of firms and evasion of indirect taxes, we show that more intense competition may have the negative side effect of increasing tax evasion, thereby, lowering public revenues and welfare. This will be the case if market entry costs decrease. A similar result will hold if marginal production costs fall and demand is either weakly concave, or convex and inelastic. The result of more competition, less evasion and higher public revenues will be obtained if (a) marginal production costs fall and demand is convex and elastic or (b) the demand elasticity increases. As a policy implication, we prove that tax enforcement should be intensified if there is a negative trade‐off between competition and evasion.