We depart from the assumption that states purposefully choose between different institutional arrangements, because they regard one or the other as more convenient for dealing with new conditions or new problems. In the case of the biosecurity domain, I argue that the choice is restricted to two institutional arrangements: transgovernmental networks (TGNs) or intergovernmental organizations. We also maintain that under certain conditions actors deliberately opt for TGNs because they consider them as superior institutional arrangements. Although states continue to rely on formal treaties to govern security cooperation, these have been increasingly complemented by a variety of TGNs. Owing to their flexibility, speed and low sovereignty costs, many anticipate that TGNs will be the preferred channel for international cooperation by performing many global governance functions but without having the form of more structured organizations. Nonetheless, as displayed in the literature review we still lack convincing answers of why states sometimes choose formal means for their international agreements. Hence, the aim of this paper is to evaluate the literature on transgovernmental networks, legalization and rational institutional design by intertwining and applying some of their theories to the biosecurity issue-area. Finally, we conclude by discussing two possible rival explanations to the rationalist approach.