A field study was undertaken on three Swiss sheep farms (A, B, C) to evaluate the efficacy of a long-acting moxidectin formulation (Cydectin® 2% LA) against gastrointestinal nematodes (GIN). Naturally infected ewes (all three farms) and their winter-borne lambs (farms A and B) were allocated to two groups (MOX, CON). At day 0 ewes of the MOX-groups were treated with 1mg moxidectin s.c. CON- and MOX-groups were grazed on separate pastures and were sampled for faeces and blood at 28-day intervals. Based on larval cultures Haemonchus contortus was the dominant GIN-species in ewes throughout the season. Over the entire observation period faecal egg count (FEC) of the MOX-ewes in farms A, B, C was 56, 84 and 87% less than the CON-ewes (p<0.05). FEC of lambs grazing with MOX-ewes was reduced in farms A and B by 56% and 61%, respectively (p<0.05), compared with the respective CON-groups. None of these lambs received anthelmintic treatment during the experiment. Therefore, the differences were due to an indirect effect mediated by the lower pasture contamination with GIN-larvae. These were reduced by 73, 81 and 74% in farms A-C respectively compared to the CON-groups (p<0.05). In farm B, where lambs remained with their mothers during the entire grazing season, these differences were also reflected by a higher daily weight gain (p<0.05) and reduced pepsinogen levels in lambs of treated ewes. This strategy offers an interesting potential for expanding refugia by propagation of GIN in untreated lambs.