Characteristic features of stomatal apparatus, i.e, stomatal density, area of the stomatal poruš, relative stomatal area and diffusion resistance (r^) were examined in young (10 d) and old (30 d) leaves of four sugar beet cultivars (Allyx, Arigomono, Monohil and Primahill). Since the plant age was also considered to be an important ontogenetic factor, all measurements were repeated at plant ages of 40, 47, 57 and 65 d. Genotypical differences among the cultivars studied could be explained in terms of the level of ploidy, i,e. an increasing number of chromosome sets was accompanied by an increase of stomatal size and a decrease of stomatal density. The other stomatal characteristics did not significantly differ among the cultivars. The increase in plant age resulted in a higher stomatal density and a gradually decreasing stomatal size. These phenomena generally induced a plant ontogeny controlled increase of the relative stomatal area of all leaf series. Young leaves showed higher stomatal densities, but the stomata were smaller and póre area was reduced by 40 to 60 %, compared to the old leaves. The young leaves also exhibited a smaller r^.