This article, based on an investigation in the agro-industrial plains of North-East France, explores the processes whereby farmers become attached to and bound by their cooperative. Inspired by the sociology of commercial relations, it analyses three interwoven aspects of farmer attachment: the institutional level, which relates to the cooperative values and principles that legitimise a call for loyalty to the cooperative project; the organisational level, with the establishment of a diverse range of services; the level of service relations, at which technical sales representatives seek to forge strong ties with producers. The article shows how the issue of attachment influences big changes in the cooperative's offering and in the work of the technical sales agents. It reveals the complexity of the cooperative relationship, the tensions between logics of loyalty and service, and the mechanisms involved in the construction of ties of dependency between farmers and technical sales agents, where it is not always clear who is dependent on whom.