Nematode infection of cattle treated in their first year at pasture with the fenbendazole slow release bolus (‘Bolus group’) was compared during the second year with that of untreated cattle. Ostertagia was the most prevalent parasite associated with Cooperia. Except for the Dictyocaulus spp. which caused clinical signs of bronchitis in the ‘Bolus’ group the infection during the second year resulted in a moderate response of the cattle whatever the group. Rises in both pepsinogen and gastrin levels were correlated with the number of Ostertagia L 3 on herbage. Damages in the abomasal mucosa were more frequent and severe in the ‘Bolus’ group where more inflammatory signs were observed in spite of a smalleer number of worms. Nevertheless, the differencesin total weight gains were not significant thanks to a compensatory effect during the second part of the grazing season in the ‘Bolus’ group. Hypotheses related to a minimum threshold of infection during the first year necessary to develop high enough protection during the second year are discussed. The pathological effects of gastrointestinal nematodes seem to vary more according to the inflammatory response than to the number of worms.