Some dandelions, Taraxacum, are diplosporous gametophytic apomicts. Crosses between closely related diploid sexuals and triploid apomicts were made to study the inheritance of apomixis. Seed-set was less than one-third of that in diploid x diploid crosses, probably because of the inviability of aneuploid pollen or zygotes. Almost 90% of the viable offspring were diploid and the result of selfing, as was shown by a discriminating allozyme marker. Aneuploid outcross pollen had a mentor effect on self-pollen, causing a breakdown of the sporophytic self-incompatibility system. A similar phenomenon has been reported before in wide crosses. Of the 26 allozyme-confirmed hybrids, four were diploids, 15 were triploids and seven were tetraploids. Diploid hybrids were significantly less frequent than triploid hybrids, suggesting either low fitness of haploid pollen or more numerous formation of diploid pollen. Emasculation and bagging of flowers indicated apomictic seed-set in none of the diploid, in one-third of the triploid and in all of the tetraploid hybrids. All apomictic hybrids showed partial seed-set, but additional cross-pollination did not increase seed-set. Cytological analysis of the F2 progeny confirmed that partial apomixis was caused by semisterility and not by residual sexuality (facultative apomixis). The difference in segregation for apomixis between triploid and tetraploid hybrids may be because the triploids originated from partially reduced diploid pollen grains, whereas the tetraploids originated from unreduced triploid pollen grains.