In 1992 governments negotiated a multilateral treaty regime to manage biological diversity. Unlike the United Kingdom, the United States rejected this treaty. Yet both nations were equally at risk from biodiversity loss and equally likely to benefit from its protection. This empirical puzzle is used to explore state choice in regulatory cooperation. Epistemic community analysis helps to explain the onset of negotiations and the contours of debates over regime norms and rules. But state choices, and the regime itself, primarily reflected the regulatory politics of biodiversity management. The international commitments on biodiversity, ostensibly alike for the U.K. and the U.S., had to be implemented through their domestic regulatory structures; the result was a distinct set of domestic ramifications. Electoral incentives and especially domestic institutions influenced both industry and governmental assessments by shaping expectations about the impact of the regime in operation. As states increasingly seek to regulate internationally, domestic institutions and anticipated implementation will play ever greater roles in explaining state choice and, because powerful states are equally influenced by these dynamics, in explaining international outcomes.