Crop diversity is essential for the development of sustainable agriculture in the face of global change. Human has exploited the great genetic diversity since the first domestication of crops. This diversity is still managed in family farming through peasant seeds, which represent more than 70% of the seeds used in West Africa. However, certified seed from genetic improvement is increasingly being distributed to improve agricultural productivity and food security. To date certified seeds and peasant seeds coexist in family farming systems. However, little is known about the coexistence modalities in the local seed network. By combining quantitative and qualitative analyses, we present a study of the circulation of millet seeds according to their certified or peasant origin by characterizing the structure, extent and modalities of seed exchanges. Based on surveys conducted among 79 farmers in the Koungheul region of Senegal, we have identified several levels of coexistence, both in the perception of varieties by farmers and in the networks mobilized to supply seeds. Our results suggest that the origin of seeds shapes farmers' seed networks. The acquisition of certified seeds from an agricultural trader also modifies the context and the terms of exchange by inducing their social isolation and limiting their self-production. Nevertheless, after harvest, certified and peasant seeds are mixed and integrated into an open and complex social network involving multiple actors. Our results provide a better understanding of the seed exchange networks mobilized by farmers to access and disseminate millet varieties. This study highlights a gap between agricultural policies and local seed management by farmers, results that could inform thinking on the governance and conservation of plant genetic resources.