Grape phylloxera (Daktulosphaira vitifoliae Fitch) is known to be a major pest for European and worldwide viticulture (Powel et al., 2013). Introduction of the pest from America to Europe in the second half of the 19th century led to the economic losses due to the destruction of vineyards. At the beginning of the 20th century American vines and their hybrids were used as rootstocks for European grapevine varieties to achieve a partial phylloxera tolerance. As a result, the hybrids showed less or no damage from phylloxera, they were initially thought to be resistant. With the increasing damage in the last few decades, it was realized that rootstocks are rather phylloxera tolerant, than completely resistant. The cases of phylloxera overcoming plant resistance leading to economic loss on the rootstock AxR1 in California are well known. With regard to feeding and development, pest occurs in two forms - phylloxera, which feeds and lives on the root (root-feeding phylloxera) and phylloxera which feeds and lives on the feeding (leaf-feeding phylloxera). Although damage by leaf-feeding phylloxera is often neglected, it does lead to reduction of photosynthesis and an additional sink tissue for the plants, it also affects the size of the population forms that inhabits and is doing the damage to the roots.The aim of this thesis was to determine the genetic diversity of grape phylloxera leaf feeding populations in Croatia and compare it with the similar data representing phylloxera genotypes from Burgenland, Austria, expecting a lot of variation within Croatian genotypes due to the diversity of the Croatian climate. The field sampling of grape phylloxera indiviudals has been conducted on the leaf infested plants, in Croatia on fall 2015. The samples were collected in DubrovačkoNeretvanska, SplitskoDalmatinska, Šibensko-Kninska, Primorsko-Goranska, Virovitičko-Podravska and Zagrebačka county. The isolation of the DNA from L5 stage of pest and the genotyping of individuals with standardized set of 7 SSR markers, and the analysis of PCR products on capillary sequencer ABI 3130 xl were conducted at the University of BOKU (Austria).In total, 120 individuals of phylloxera were genotyped and analyzed. A high genetic variability of Croatian populations was assessed, especially among individuals of the coastal area. The results confirmed a high proportion of individuals resulting from asexual reproduction (1/3), but among them the dominant genotype (clone) was not observed.