To avoid disease outbreaks caused by gastro-intestinal nematodes different prophylactic measures are recommended. In this connection pasture hygienic achievements and anthelmintics are used. In practice it is often difficult to realize pasture hygienic steps as move the animals to parasite free areas and to avoid that they come back to the infected. In many cases the parasite control is limited to routine deworming. Our intention was to study if anthelmintic treatment without other measures had any effect on weight gain and number of eggs in faeces in calves naturally infected with Ostertagia ostertagi. The studies were done during two stable seasons 1975/76 and 1976/77 when the calves were treated two weeks after stabling, and during two grazing seasons 1976 and 1977 when the calves were treated 20 days or 15 and 20 days after turning out. In none of the trials any obvious influence on the weight gain and number of eggs was observed. The failing effect during the stable season might depend on the fact that the parasites to more than 90% were in inhibited L4-stage, and the failing effect during grazing, is surely an effect of reinfection when the calves remain on the infected pastures. As prophylactic measure anthelmintic treatment is of very little value if not the calves at the same time are protected against reinfection by moving to clean pastures.