The aim of this work was to determine the effects of 20, 25 and 30 minute UV-irradiation periods lambda = 253.5 nm to two planarian species Dugesia tigrina (Gir.) and Polycelis felina (Daly.). In vivo, UV light effects have been reported to affect intracellular receptors and disrupt simple behaviour. The effects of UV-rays on mortality and behavior as well as morphological, cytological and histological changes in the two planarian species were assessed, and the course and the dynamics of regenerative processes were compared between them. Experimental populations of Dugesia tigrina and Polycelis felina species were maintained in laboratory conditions at room temperature. Mortality, behavioral and morphological changes were monitored daily by means of a light stereomicroscope. For cytological and histopathological analysis, planarians were fixed in Bouine fixative on the first, second, third, fifth and seventh day after exposure to UV-irradiation, respectively. They were embedded in paraffin, cut on a microtome, stained with toluidin blue and embedded in Canada-balsam. UV-rays caused mortality, behavioral, morphological, cytological and histological changes in each planarian species. In regeneration of damaged body parts reticular cells and neoblasts played the main role. Neoblasts as totipotent cells extremely increased in number in the area of damaged tissue, immediately after UV-exposure. Dugesia tigrina was more sensitive to UV-rays than Polycelis felina due to possession of less pigmented cells. The course of regeneration in both species was similar. Most individuals of both species regenerated in 5 to 12 days after UV-irradiation.