Regulators often do not regulate all firms competing in a given sector. Due to productsubstitutability, unregulated competitors have incentives to bribe regulated firms to have themoverstate their costs and produce less. The best collusion-proof contract entails distortions bothfor inefficient and efficient regulated firms (distortion ‘at the top’). But a contract inducingactive collusion may do better by allowing the regulator to ‘team up’ with the regulated firmto indirectly tax its competitor. The best such contract is characterized. It is such that theunregulated firm pays the regulated one to have it truthfully reveals its inefficiency.