This paper compares the new institutional economics literature on agricultural cooperatives to their actual operation in Greece. The key contrast thus revealed concerns the role of opportunism. New institutional economics explains cooperatives in terms of their ability to combat external opportunism, subject to the constraints of internal opportunism. In contrast to this literature, in Greek agricultural cooperatives, internal opportunism is shown to inflate their formal organizational structures and to lower their effectiveness in serving members. The paper concludes by discussing theoretical, policy and practical implications of this argument.