Background. The study was carried out in the shallow, eutrophic Oświn Lake, in the area of the Seven Islands ornithological reserve (north-eastern Poland) to determine the infection of fish with digeneans maturing in piscivorous birds. This paper presents a comparative analysis of the occurrence of Posthodiplostomum cuticola in fish from two parts of the reservoir differing in environmental conditions. The effect of fish size and season of sampling on the infection level was analysed.Materials and methods. Within 1998 and 1999, a total of 1091 fishes of 8 species dominating in the lake were examined. Samples were collected four times a year (in May, July, August, and October) simultaneously from the two examined parts of the lake.Results. Metacercariae of P. cuticola occurred with a high prevalence and intensity of infection in roach, rudd, white bream, and carp bream, and sporadically in crucian carp and European perch. The intensity of infection of fish from the western part of the lake was significantly higher compared with the eastern part; roach from the western pool were much more infected than in the other reservoirs of north Poland. The infection of roach, white bream and carp bream increased with total body length (TL), such a relationship was not reported for rudd. The infection of roach, white bream and carp bream varied significantly in different experimental periods while differences in the infection of rudd in samples were incidental.Conclusion. Location-dependent occurrence of P. cuticola in fishes indicates that interchange between fish groups from the eastern and western pool of Oświn Lake is limited. Infection of rudd was of a more incidental character then the infection of roach, white bream, and carp bream.

parasite, fish, Posthodiplostomum cuticola, Oświn Lake, "Seven Islands" reserve, Poland