In recent decades, policies in many countries have been shaped by the implementation of economic liberalization, characterized by state withdrawal from marketing operations and control of trade. In this era of liberalization, policies regulating commodity marketing and trade were expected to disappear, but, in fact, this has hardly occurred. A comparative study is carried out of three farm sectors in Costa Rica, based on a dynamic analysis of behaviour in the context of the institutional change brought about by liberalization. The capacity of farmers' organizations to adapt and contribute to institutional change through their control over economic activity and their participation in policymaking processes is highlighted. In so doing, it is shown that, even in a liberalized era, policies regulating marketing and trade still exist and affect the functioning of agricultural markets. These policies differ according to the farm sector and can be linked directly to the influence of farmers' organizations within these sectors. It is shown that organizations play a key role in the regulation of farm sectors, and that their success depends on the institutional and organizational "thickness" to which they have contributed in each sector. Differences in historical trajectories can explain differences in the capacity of organizations to influence policymaking and to gain market share.