Because domestication rarely leads to speciation, domesticated populations often hybridize with wild relatives when they occur in close proximity. Little work has focused on this question in clonally propagated crops. If selection on the capacity for sexual reproduction has been relaxed, these crops would not be expected to hybridize with their wild relatives as frequently as seed-propagated crops. Cassava is one of the most important clonally propagated plants in tropical agriculture. Gene flow between cassava and wild relatives has often been postulated, but never demonstrated in nature. We studied a population of a wild Manihot sp. in French Guiana, which was recently in contact with domesticated cassava, and characterized phenotypes (10 morphological traits) and genotypes (six microsatellite loci) of individuals in a transect parallel to the direction of hypothesized gene flow. Wild and domesticated populations were strongly differentiated at microsatellite loci. We identified many hybrids forming a continuum between these two populations, and phenotypic variation was strongly correlated with the degree of hybridization as determined by molecular markers. Analysis of linkage disequilibrium and of the diversity of hybrid pedigrees showed that hybridization has gone on for at least three generations and that no strong barrier prevents admixture of the populations. Hybrids were more heterozygous than either wild or domesticated individuals, and phenotypic comparisons suggested heterosis in vegetative traits. Our results also suggest that this situation is not uncommon, at least in French Guiana, and demonstrate the need for integrated management of wild and domesticated populations even in clonally propagated crops.