Thiabendazole was the first of several benzimidazole anthelmintics and was superior to previously available drugs in both-efficiency and safety. Of the “second generation” of benzimidazoles, mebendazole and cambendazole are widely used inhorses and provide wider efficacy spectra and lowerdose rates compared with thiabendazole. Thiabendazole has proven activity against larval strongyles, but requires high dose rates. Two organophosphates, dichlorvos and haloxon have been formulated as horse anthelmintics. Pyrantel and its analogue morantel, are reported as efficient equine anthelmintics, but only in-feed and pasteformulations are recommended by the manufacturer as suitable for horses. Strains of small strongyles tolerant to benzimidazoles have been reported in U.S.A. and U.K. The addition of a piperazine salt to approximately half recommended dose rate, or the use of pyrantel, removed the tolerant nematodes.