The number of cercarial dermatitis (swimmer's itch) cases in the Nordic countries has been increasing during the latest years. Thus, records from Iceland, Norway, Sweden and Denmark have occurred at increasing frequencies. In order to elucidate the biological basis for this infection of ten species of pulmonate snails in some Danish lakes were monitored in 2003. Ocellate cercariae belonging to the genera Trichobilharzia and Bilharziella were recorded in 1.3% and 0.71% of all snails examined, respectively. However, in specific snail hosts the prevalences recorded were up to 50%. The biotic and abiotic factors responsible for the observations are discussed, but the relatively high prevalence of infected snails in the present study calls for further monitoring of these pathogenic cercariae in European freshwater localities.