I investigate the optimal auditing scheme for a revenue‐maximizing tax collection agency that observes not only reported profits, but also a single factor of production at each firm. Each firm is owned by a single entrepreneur whose managerial ability is random. The optimal auditing scheme is discontinuous and nonmonotone in ability. In intermediate audit costs, less productive entrepreneurs face auditing probabilities that increase in ability, whereas the ablest ones are not audited. Finally, the effective tax rate is higher in the middle of the managerial ability distribution; thus, the overall regressive (or progressive) bias that arises from evasion is unknown.