The efficacy of moxidectin was evaluated in young ponies naturally infected with gastrointestinal parasites. Eight animals were treated orally with moxidectin at 0.4 mg/kg bodyweight and eight received only the vehicle. They were all necropsied two weeks later. Faecal samples were examined daily for egg counts and larval cultures. Parasites were recovered from total faecal samples collected daily and from the gastrointestinal tracts at necropsy. Moxidectin reduced the strongyle egg counts by >99 per cent from three days after treatment but some individuals remained positive for 10 days. The drug had little or no ovicidal activity. As evaluated in the critical-controlled test, moxidectin was 99 to 100 per cent effective (P<0.05) against luminal stages of Parascaris equorum, Strongylus vulgaris, Triodontophorus species, Craterostomum acuticaudatum, 19 cyathostome species and Oxyuris equi. Adult S edentatus were also completely removed. Its efficacy against third stage larvae of Gasterophilus intestinalis was 95 per cent (P<0.05). Luminal nematode stages were removed within a few days, and bots continued to be eliminated for at least two weeks after treatment. No activity was observed against Anoplocephala perfoliata. As evaluated in the controlled test, moxidectin was 100 per cent effective against Habronema muscae (P<0.05) and had a 76 per cent but not significant efficacy against encysted small strongyle larvae.