In the U.S. defense procurement market, regulators require contractors to make “mandatory disclosures” if principals at those firms determine, after due review, that there is credible evidence that the firms engaged in certain crimes (fraud, bribery or gratuities), civil fraud, or significant overpayment by the government. Failure to make such a mandatory disclosure, required by clause and by regulation, can lead to (among other things) the debarment of the contractor -- a potentially devastating result. Mandatory disclosure is a natural extension of a separate requirement, that contractors maintain effective corporate compliance and ethics systems, and the Defense Department’s largest prime contractors, with sophisticated compliance systems in place, have been able to accommodate the mandatory disclosure requirement. This paper asks whether this disclosure requirement in effect favors those largest contractors, and decreases competition in a already highly concentrated defense market, either by creating substantial legal risks for firms too small or inexperienced to institute effective compliance and disclosure systems, or by discouraging competition from other companies in the commercial sector. The paper concludes that the mandatory disclosure rule can impair competition in defense procurement, and recommends that regulators carefully shape any disclosure requirements, and perhaps reconsider relying on voluntary disclosure, mindful of the need to reduce costs and enhance competition in defense procurement markets.