The questions that link agriculture and the conservation of nature are so diverse and so complex that it would be unthinkable to reduce them to a corpus of rough-and-ready data directly useable by those in charge of dealing with them. There are many situations where the different actors operating in the countryside have to consult each other so as to conceive adequate means of management ; they do so by confronting their different points of view, by sharing and exchangeing their knowledge, both as experts and as laymen, and by assessing the efficiency of their work all along its progress. The authors start from a simple case involving two farmers and one nature scientist and use the results of two case studies in the marshlands of western France in order to analyse how such devices for concertation are working. Their first aim is to specify where and how dialogues can take place among the various people involved in farm environmental questions, in other words, they tackle the problem of the forms and sizes of the meetings, which do affect the process of co-construction. Their next step is to elaborate 'cognitive equivalences', i.e. to set up references against which the values, knowledge and norms can be confronted, as a result of the interaction. Their analyses bear specially on the adjustments of the statements put forward, called here 'socio-technical' and on role played by expertise and by the tools used to formalize it in these adjustments.