The competitive quest of the Cartagena Protocol and the WTO for authority to regulate international trade in genetically modified organisms (GMOs) exemplifies a typical interaction between international institutions with diverging objectives. This article first develops a conceptual framework for the analysis of institutional interaction that emphasizes disaggregation of complex interaction situations into separate cases of clearly directed inter-institutional influence. These cases can follow different causal mechanisms. Second, applying this framework to the interaction between the Cartagena Protocol and the WTO reveals that existing commitments have driven parties toward a step-wise delimitation of the institutions' jurisdictions. Although the WTO acquired a firstmover advantage by structuring the regulatory field, the Cartagena Protocol showed surprising strength in exploiting the remaining room for maneuver. The structure of international governance thus steers institutions with differing objectives toward a jurisdictional balance that, while reflecting existing power relations, limits the potential for conflict and frames available policy choices.