A study was conducted to investigate reinfection with nematodes in calves following treatment with doramectin or ivermectin administered subcutaneously at a dose rate of 200 micrograms kg-1 of body weight under conditions of natural challenge. Thirty calves were allocated to three groups of 10 calves each based on body weight, sex, breed and pre-treatment faecal egg counts (FEC) after grazing together on a common pasture for three months. Treatments were doramectin, ivermectin and no treatment. Calves were returned to the same pasture for 56 days, placed on dry lot from days 56 to 63 and then necropsied over days 64-66. Faecal egg counts were done at days -1 and 0, then bi-weekly from day 14 to 56 and day 63. Mean FEC in control calves continued to rise throughout the grazing period. Trichostrongyle FEC were significantly (P < 0.05) greater in the control group compared to either treated group at each sample time following treatment. At necropsy, a total geometric mean of 19,847 nematodes per calf was recovered from untreated controls, of which eight genera were identified. The predominant nematode genera were Ostertagia (8749), Nematodirus (3702) and Cooperia (1927). In the ivermectin-treated calves, geometric mean worm burden was similar to that of the untreated controls: A total of 20,349 nematodes per calf was present including the genera Nematodirus (8633), Ostertagia (4700) and Cooperia (1740). In contrast, the geometric mean worm burden in doramectin-treated calves was 12,173, including Ostertagia (4310), Cooperia (1141) and Nematodirus (1667). Doramectin was more effective than ivermectin in protecting calves from reinfection over a 56-day post-treatment grazing period under conditions of natural challenge as measured by accumulated mean worm burdens.