In a controlled trial with 32 calves on the persistence of the effect of a 0.5 mg/kg moxidectin pour-on against cattle nematodes the following periods of persistence were observed: Virtually 100 per cent efficacy for more than five weeks against Dictyocaulus viviparus (total numbers and juveniles) and Ostertagia species (total numbers, inhibited early fourth stage larvae [L4] and juveniles); over 97 per cent for more than four weeks against Trichostrongylus axei (total numbers, inhibited early third stage larvae [L3] and juveniles); 100 per cent for more than two weeks against T vitrinus (total numbers and inhibited early L3), and more than 90 per cent against Cooperia species (total numbers and inhibited early L4), and 89 per cent against L4 of Nematodirus species. Moxidectin thus had a more persistent effect against worms from the lungs and the abomasum than against worms from the small intestine, particularly because differences were observed between Trichostrongylus species from both locations. The population dynamics of T axei and T vitrinus in the calves appeared to differ from that in sheep. The relatively high percentages of inhibited early L3 in the untreated calves suggest that there was a much stronger tendency for a seasonal pattern of inhibition than in sheep.