The role of nonstate actors in international environmental politics has been given increased scholarly attention during the last decade. While most analyses are focused on direct nonstate influence at the international level, one main objective of this article is to develop a multi-level approach that allows analysis of nonstate influence channeled via the domestic decision making level. The point of departure for the analysis is the International Whaling Commission (IWC) during the period from 1970 to 1990, with a particular focus on the competition for influence characterizing the relationship between the scientific community and the environmental and animal rights movement. The analysis shows that domestic channels of influence may be equally, or even more important than channels of influence linked to the international decision making level. In the case of the IWC, for instance, the environmental and animal rights movement succeeded in mobilizing domestic public support, particularly in the United States, and had a key ally in the US government, Congress and Administration. The domestic role of this nonstate actor was of key importance to its success in influencing the development of the international whaling regime. The analysis shows, therefore, that examining the role of the domestic channel is integral to understanding nonstate influence on international policy-making, and particularly how some nonstate actors acquire influence at the expense of others.