In 2011-2013, a study was conducted on the colonization and damage to the leaves and shoots of plum trees by the plum rust mite Vasates fockeui (Nal.) and on the damage to the leaves of pear trees by the pear-leaf blister mite Eriophyes piri (Pgst.). Observations were carried out in the Experimental Ecological Orchard of the Research Institute of Horticulture, located in Nowy Dwór-Parcela near Skierniewice. Colonization of leaves by the plum rust mite was checked four times during the growing season, from May to August. In late July or early August, there was executed an assessment of the damage to the leaves and shoots of 4 plum cultivars (‘Herman’, ‘Żółta Afaska’, ‘Čačanska Rana’ and ‘Valjevka’) and 6 pear cultivars (‘Faworytka’, ‘Radana’, ‘Konferencja’, ‘Erika’, ‘Concorde’ and ‘Amfora’). The damage was assessed on a 4-point grade scale, where 0 - meant no symptoms of feeding by the mites, and 3 - severe damage (90% discoloured leaves of pear trees, and premature drop of plum tree leaves). The highest degree of damage to plum trees by the plum rust mite was recorded in the cultivar ‘Herman’, and the lowest in ‘Valjevka’. The average extent of grade 3 damage to shoots was 31.8% and 2.7%, respectively. On pear trees, leaves most severely damaged by the pear-leaf blister mite were observed in the cultivars ‘Amfora’ and ‘Erika’, while the lowest degree of damage was shown by the cultivars ‘Radana’ and ‘Konferencja’.