In order to establish the infection patterns in the first intermediate host of Dicrocoelium dendriticum, investigations were carried out on sheep pastures situated in the eastern part of Brandenburg, Germany, during the grazing periods in 1986, 1987, 1989 and 1990. It was found that the majority of specimens of the first intermediate host, Helicella obvia, become infected in the autumn of their second year of life, when their shell diameter is of medium size. The percentage of snails containing daughter sporocysts was highest in spring. Slimeball output could be provoked only in May and June.