The five cultivated species of Cucurbita (C. pepo, C. mixta, C. moscbata, C. ficifolia and C. maxima) are very much alike in their morphological characteristics, but they are for the most part cross-incompatible. They have apparently developed a sequence of sterility barriers that effectively prevent gene flow between them. In a series of interspecific matings, the five cultivated species are shown to be cross-compatible with a wild species, C. lundelliana, from southern Mexico and neighboring Central America. Matings between C. lundelliana and C. moschata, C. maxima and C. ficifolia produced self-fertile F₁ plants that were cross-fertile with either parent. The other two matings (C. lundelliana × C. mixta and C. lundelliana × C. pepo) produced fruit, but the seeds had small embryos. The data from interspecific hybridization and those of geographical distribution suggest that C. lundelliana may have had an important role in the origin of the cultivated group.