Netobimin was tested for efficacy against Haemonchus contortus using 7 groups of 5 parasite‐free lambs of six month age. The lambs in group 1 and 2 were infected with 10,000 larvae of a benzimidazole susceptible strain and those in groups 3–7 with the same dose of a resistant strain. The following treatment scheme was applied 21 days after infection: lambs in groups 2 and 4 ‐ 7.5 mg kg‐1netobimin, in group 5 ‐ 20 mg kg‐1netobimin, in group 6 ‐ 5 mg kg‐1oxfendazole and in group 7 ‐ 3.8 mg kg‐1albendazole. The lambs in groups 1 and 3 remained untreated. All lambs were slaughtered 28 days after infection. Egg counts decreased in all lambs after treatment, but increased again in lambs in groups 4, 6 and 7. There was a slight increase in lambs in group 5, while those in group 2 remained negative. Post‐mortem worm counts showed a reduction of 99.8 per cent in lambs in group 2 compared to those in group 1. In lambs in group 4–7 the reduction of worm counts was respectively 40.9, 89.5, 24.7 and 40.7 per cent compared to those in group 3. Egg development assays carried out 20 days after infection showed an average LD50of 0.46 mg ml‐1thiabendazole for the resistant strain. After treatment (day 27) the LD50was 0.53, 0.48, 0.58, 0.56 and 0.47 in lambs in the groups 3–7. It is concluded that netobimin and other (pro) ‐benzimidazoles should not be used in cases of benzimidazole resistance and that levamisole, pyrantel tartrate or ivermectin are preferable.