In the alpine region, mixed grazing systems with cattle of different age, origin and prophylaxis against trichostrongylid infections are most common. Under these conditions the administration of anthelmintic devices to susceptible calves is frequently postponed to June or July to achieve a better protection during the period of increased pasture infectivity in summer and autumn. In a field experiment with 27 first-year grazing calves a morantel sustained-release trilaminate bolus (MSRT, Pfizer) was given to two groups (Groups A and B) of nine naturally infected calves each, on 22 July. Calves of Group B were moved to a clean pasture (B) 1 day later, whereas the calves of Group A remained on the previous pasture (A) together with nine untreated calves (Group C). The contamination with infective larvae (L3) on Pasture A remained below 1000 L3 kg-1 dry matter, which was sufficient to produce clinical parasitic gastroenteritis in five of nine control calves. The MSRT bolus reduced the mean egg output by more than 90% within 14 days after administration and prevented clinical parasitic gastroenteritis in the calves of Groups A and B. Owing to the persisting infection risk on Pasture A, the mean serum pepsinogen levels reached about 3000 mU tyrosine in the calves of Group A in September compared with approximately 2000 mU in Group B grazing the clean pasture. However, the differences in pasture contamination were not reflected in the mean bodyweight of the calves, which were 20 kg heavier at the end of the trial in both MSRT-treated groups compared with the control calves (P < 0.01). As there is a high probability that a moderate larval contamination is present on prealpine and alpine community pastures in summer, the metaphylactic use of an MSRT bolus in mid-July is likely to limit trichostrongylid infections within a subclinical range and thus provides sufficient protection of susceptible calves against parasitic gastroenteritis.