We study collusion among firms against imperfectly monitored environmental regulation. Firms increase variable profits by violating regulation and reduce expected noncompliance penalties by violating jointly. We consider a case of three German automakers colluding to reduce the effectiveness of emission control technology. By estimating a structural model of the European automobile industry from 2007 to 2018, we find that the collusion lowers expected noncompliance penalties substantially and increases buyer and producer surplus. Welfare decreases by € 0.73–2.51 billion because of increased pollution. We show how environmental policy design and antitrust play complementary roles in preventing noncompliance.