The cohesion of interest associations has been seen to depend, among other factors, on the character of the political regime. Federal regimes are said to have negative consequences for group cohesion because of their tendency to force extensive regional differentiation in associations. These conclusions are examined here and shown to be in need of elaboration. To explain differentiation in business associations, factors other than regime structure must be considered, specifically the structure of the sector being represented and the nature of the collective bargaining arrangements. When this is done, the wide variety of internal regional arrangements found in associations is better accounted for. Furthermore, some of these arrangements are shown to be more likely to weaken cohesion than others. The argument is based on a study of business associations in Canada and the United Kingdom that are active in three economic sectors—food processing, chemicals, and construction.