In 1975, a number of genetic lines discovered in our maize genetics nursery in Ames, Iowa, showed unidirectional cross-incompatibility. Later, it was found that this unidirectional cross-incompatibility is controlled by three recessive genes. One locus (cif) controls the incompatibility reaction in the female tissue and the other two (cim1 and cim2) control the incompatibility reaction in the pollen grain. The cross is incompatible only when the female parent is homozygous recessive for the cif and the male parent is homozygously recessive for the cim1 as well as the cim2 locus. Cytological studies of this unidirectional cross-incompatibility show that the site of the incompatibility reaction occurs after the entry of the pollen tubes into the transmitting tract of the incompatible silks. Between 12 and 24 h after pollination, the incompatible pollination is characterized by the swelling and bursting of pollen tubes at the tip, after which pollen tube growth stops.Key words: maize, pollen tube, cross-incompatibility.