Collective management of cropping systems is common in tropical countries when one production factor (e.g. machinery or an irrigated scheme) is shared by a number of farmers. To achieve their technical objectives in these contexts, farmers have to co-ordinate decision-making processes among themselves and with their economic partners. A 3-year study was carried out on two irrigated schemes in the Senegal river delta. Its aim was to understand (1) the problems farmers'organisations managing schemes faced when carrying out annual double cropping of rice and (2) how they managed to co-ordonate the different actors (individual farmers, contractors and collective organizations) interacting on these schemes. The results presented here relate mainly to the harvest part of the problem. They show double cropping success varies from one site to another and from one year to another, depending on different starting dates and global harvest performances. The comprehensive model proposed to eplain the diversity includes (1) analysing individual farmers' and contractors' decision-making processes [...] and (2) classifying collective co-ordination processes under three main strategies of contractualizing relations [...]. In the discussion the authors propose a general framework to explain the co-ordination problems met by farmers in this context including lack of experience, diversity of individual behaviour and uncertainty