There is widespread concern that a number of U.S. markets have become more concentrated and less competitive in recent years. This development suggests an enhanced role for antitrust enforcement to protect and promote competition in these markets. But there are limits on the anticompetitive conduct antitrust enforcers and private plaintiffs can reach, especially in regulated markets. Two Supreme Court cases from the 2000s, Verizon Communications v. Law Offices of Curtis V. Trinko and Credit Suisse v. Billing appeared to enhance these restrictions, increasing the likelihood that regulation will displace antitrust entirely, especially in the financial sector. As a result, the job of confronting heightened concentration and reduced competition in financial services and other regulated markets may fall to sector regulators. These agencies are unprepared for the task and often are unwilling to undertake it. They have neither the resources nor personnel to enforce competition rules and such enforcement ranks low on their priority list. Competition in the financial markets (and other regulated markets) therefore may suffer. The stakes are high: increased concentration in financial markets harms consumers and may threaten systemic safety. This Article proposes a regulatory-design solution to the problem and focuses on Dodd-Frank's regulatory regime for the derivatives markets as a case-study. It argues that sector regulators should craft structural rules to protect competition in these markets-including ownership and governance restrictions on derivatives clearinghouses and exchanges-rather than solely relying on conduct rules and corrective measures taken ex post. The Article contends that increased reliance on regulatory responses to competition problems in regulated markets may (surprisingly) be beneficial from a competition standpoint and that these salutary effects may be enhanced when the products involved are potentially toxic, as is the case for some derivatives products. These conclusions are applied more broadly to other concentrated regulated markets.