Increased market concentration of firms is found to be directly related to pollution control effort, after controlling for firm size, profitability, and pollution potential. The theoretical model reveals that the firm selects product output and waste control inputs in order to minimize pollution abatement costs. Increasing rivalry lowers the opportunity cost of pollution abatement and reduces profits. This causes substitution effects between product output and waste control inputs, and scale effects, both of which may reduce pollution control efforts. The empirical implications are tested using four-digit data for US manufacturing industries.