The susceptibility of a known thiabendazole-resistant population of small strongyles to anthelmintics of both benzimidazole and non-benzimidazole groups, was determined. In the first study, 42 horses infected with thiabendazole-resistant small strongyles were allocated to 6 groups. Treatment groups received one of the following anthelmintics: mebendazole, febantel, febantel plus trichlorphon, morantel tartrate, or a combination of thiabendazole, piperazine and trichlorphon. Morantel tartrate and the thiabendazole/piperazine/trichlorphon combination produced highly significant (p less than 0.001) reductions in faecal strongyle egg counts 20 days post-treatment. Mebendazole, febantel and febantel plus trichlorphon failed to reduce strongyle egg counts significantly. Larval culture and differentiation indicated that in all cases of anthelmintic failure, small strongyles of the sub-family Cyathostominae were involved. Eighteen horses from groups in which treatment had failed were re-allocated to 3 groups. Treatment with either morantel tartrate or haloxon was highly efficient in reducing faecal strongyle egg counts. In the final study, fifty-four horses, infected with benzimidazole-resistant small strongyles were allocated to 10 groups. On day zero, each treatment group received one of the following anthelmintics: thiabendazole, cambendazole, mebendazole, oxibendazole, piperazine, thiabendazole/piperazine, cambendazole/piperazine, mebendazole/piperazine or oxibendazole/piperazine. Oxibendazole, piperazine and the benzimidazole/piperazine combinations produced highly significant reductions in faecal strongyle egg counts 20 days post-treatment (p less than 0.001). When administered alone, benzimidazole anthelmintics failed to reduce strongyle egg counts significantly, with the exception of oxibendazole. Larval culture and differentiation indicated that in all cases of anthelmintic failure, the species involved were small strongyles of subfamily Cyathostominae. There was no significant increase in benzimidazole resistance level (based on in vitro assay) as a result of drug treatment, over one generation.