WTO rules prohibit "disguised protection" in the form of domestic policies. How then do governments cooperate over trade and domestic policies when none can verify whether a nation's domestic tax reduction is a protective measure or a reaction to a production externality? In this paper, each government privately observes whether a production externality associated with its import-competing good is high or low. This paper finds that in an optimal agreement, disguised protection with domestic policies is never used by governments with a high externality and is never commonly realized. Moreover, in an optimal agreement, tariffs may be conditional on domestic policies. (c) 2006 Elsevier B.V. All rights reserved.