The 13 species ofAesculusrecognized in a revision by Hardin (1957, 1960) and two species ofBilliawere investigated by LM and SEM, and three species ofAesculusby TEM. In addition to the species recognized by Hardin four more species ofAesculus (A. bushii, A. megaphylla, A. rupicolaandA. polyneura) can be distinguished on the basis of their pollen characteristics. Four pollen groups ofAesculusspecies are described. The data of Hsiech (1979) on the polymorphism of the sect.Calothyrsuswere confirmed. In addition to a few pollen features which are discrete one, a continuum or more complicated regular varieties was observed with respect to many pollen characteristics. These patterns are not reflected in any typology or classification and are usually ignored in systematic investigations, and consequently also in the evolutional interpretation of the morphological data. This investigation of the regular pattern in the variety of pollen aperture arrangement allows an assumption about the mechanisms of pollen aperture determination and the origin of variety.