In order to confirm benzimidazole resistance as recommended at a workshop of the Commission of the European Communities the isolate 'E' of cyathostome strongyles originating from a stud where benzimidazole resistance had been demonstrated by egg hatch tests and by egg count reduction tests was investigated in two series of critical tests. Each of 11 foals reared strongyle-free was infected with 130,000 third stage cyathostome larvae. One animal remained untreated, two pairs of foals were treated with paste formulations of the (pro)benzimidazoles cambendazole (20 mg/kg bodyweight) or febantel (6 mg/kg bodyweight) and two groups of three foals were given pastes containing the non-benzimidazole drugs pyrantel pamoate (19 mg/kg bodyweight) or ivermectin (0.2 mg/kg bodyweight) either at 101 days (trial 1) or at 59 to 62 days (trial 2) after infection. Strongyles were counted in faecal samples collected daily between treatment and post mortem examination five or seven days later and worm burdens were counted in the intestinal contents and mucosal digests. Nine species of the cyathostome subfamily were found in the infected foals. The numbers of luminal stages were reduced by only 3.1 and 20.2 (mean 7.9) per cent by cambendazole and by 13.6 and 32.8 (mean 21.3) per cent by febantel in the individual animals. However, pyrantel pamoate (93.6 to 98.2, mean 96.3 per cent reduction) and ivermectin (100 per cent reduction) were highly effective. These trials provide the first report of benzimidazole resistant Cylicostephanus poculatus anywhere in the world and demonstrate (pro)benzimidazole resistance in seven other species for the first time in Europe.