Five experimental plots of 3 x 8 m containing 80 plants each, 60 of Solarium stenotomum (12 clones) and 20 of S. sparsipilum (unknown number of clones) interspaced at random, were established at the Granja K'ayra in Cuzco, Perú. The plantings were intended to mimic small fields of traditional farmers on low fertility soil and steep terrain. Plants of each species were distinguished from another by at least four isozyme loci and by stem pigmentation. Ninety five percent of the seedlings grown from either open-pollinated seed of S. stenotomum or S. sparsipilum were found to be interspecific hybrids. That the majority of the open-pollinated seed were of hybrid origin suggests that gene flow between weedy and diploid cultivated species is important in generating new landraces of potatoes in the Andes. © 1990 Springer.