Despite the major ecological and economic impacts of gene fl ow between domesticated plants and their wild relatives, many aspects of the process, particularly the relative roles of natural and human selection in facilitating or constraining gene fl ow, are still poorly understood. We developed a multidisciplinary approach, involving both biologists and social scientists, to investigate the dynamics of genetic diversity of a sorghum weed - crop complex in a village of Duupa farmers in northern Cameroon. Farmers distinguish a gradient from weedy morphotypes ( naa baa see , haariya , and genkiya ) to domesticated morphotypes; haariya and genkiya have intermediate morphological traits. We investigated the pattern of diversity in this complex using both morphological and genetic data. Our biological results are interpreted in the light of data on farmers ' taxonomy and practices such as spatial pattern of planting and plant selection. Both morphological and genetic data are congruent with farmers ' taxonomy and confi rm the introgressed status of intermediate weedy morphotypes. Farmers actively select against weedy morphotypes, but several practices unconsciously favor gene fl ow. Furthermore, haariya and genkiya may facilitate introgression between naa baa see and domesticated morphotypes by virtue of their intermediate fl owering period and their mode of management by farmers.