The efficacy of an isolate of the nematophagous fungus Duddingtonia flagrans against gastrointestinal nematodes of cattle was examined at 2 dose levels on 2 permanent pastures, with high and low stocking rates, respectively. Thirty calves, experimentally infected with Ostertagia ostertagi, were divided into 3 comparable groups and allocated to 3 similar paddocks in each of the 2 trials. Two of the 3 groups received fungal material once per day during the initial 2 months, either at high dose (106 fungal spores/kg body weight) or low dose (5×105 or 2·5×105 fungal spores/kg body weight). The third group remained as an untreated control group. Faecal, blood, and herbage samples were collected and animals were weighed every month from May to September. The pasture grazed at a high stocking rate had a large number of overwintering infective larvae, while the pasture grazed at a low stocking rate had a low overwintering herbage larval infectivity. The results showed that, at a high stocking rate, the recovery of infective larvae on pasture was diminished and calves were prevented from clinical ostertagiosis by using the D. flagrans Troll A-isolate. At low stocking rate, the parasite burden seemed not to be very heavy, and a conclusive effect of the fungi at the dose-level used could not be detected.