The persistence of the efficacy of moxidectin 0.5 per cent pour-on and moxidectin 1 per cent injectable against Ostertagia ostertagi and Dictyocaulus viviparus in calves was studied in two experimental trials. In the first trial two groups of seven calves were treated with either the pour-on or the injectable formulation, while a third group remained untreated. All the animals were infected daily from Monday to Friday with infective stages of O ostertagi and D viviparus between the day of treatment (day 0) and day 33, and were necropsied for worm counts three days later. The experimental design of the second trial was similar to that of the first but the period of infection was from 28 to 45 days after the treatment, and the necropsy was five days after the last infection. In both trials both moxidectin formulations had very high efficacies (99.6 per cent) against adult and developing stages of 0 ostertagi and D viviparus. The higher efficacy of the moxidectin pour-on preparation against early fourth stage larvae in both trials suggested that its effect was more persistent. It was calculated that the efficacy of moxidectin against O ostertagi persisted for at least five weeks for the injectable formulation and six weeks for the pour-on. The efficacy of moxidectin against D viviparus lasted for at least six weeks for both formulations.