EN
Sixty two specimens of the water snake, Natrix natrix L., from various regions of Poland, have been examined yielding 21 species of parasites: 6 species of adult trematodes, 5 meso- and metacercariae, 1 cestode larva, 5 adult nematodes, 2 nematode larvae, 1 acanthocephalan and 1 tick. Most of the adult trematodes and one nematode are specific parasites of water snakes, the other species occur more-or-less commonly in the other reptiles; some are typical parasites of amphibians, being only occasional parasites of water snakes. Adult parasites inhabited the alimentary tract and lungs, the larvae were usually encysted in the body cavity. Some differences in the levels of infections of snakes from various regions of the country were noted as well as differences related to the host age and sex. Interpretation of these phenomena is based on the behaviour of the snakes. It is also suggested that the decrease in the number of intestinal parasites, observed in older snakes, is caused by bufotoxin from the skin of toads, occasionally devoured by the snakes.