My study focuses on questions of power, control and resistance within worker co-operatives. In a first part, I analyse a situation involving interactions between representatives of the co-operative movement so as to show the role played by critique in their understanding of their pluralistic environment. The remainder of my work draws on the ethnographic study of a co-operative sheet-metal factory with some thirty workers-owners, in which I have worked as an operative during one year. Within this co-operative, I evidence the presence of two main forms of control. The first draws on bottom-up mechanisms, through which co-operators undermine the power of their chiefs so as to insure the democratic functioning of the organisation. The second is a form of peer-control, based on craft ethics, which co-operators rely on in order to prevent the managerialisation of their organisation. My research work thus contributes to show the importance of studying alternative forms of organisation such as co-operatives for furthering our understanding of questions of power, control and resistance as well as the way in which a strong professional culture can serve as an impediment to processes of co-operative degeneration.