The anthelmintic activity of cambendazole 31% paste was evaluated in a controlled trial against naturally acquired gastro-intestinal parasites of the horse. Four groups of five adult horses were treated at dose rates of 0, 15, 20 and 25 mg/kg. The three dose levels of cambendazole tested were efficacious against the large strongylesStrongylus edentatusandS. vulgaris, the stomach worm Trichostrongylus axei, the pinwormsOxyuris equiandProbstmayria viviparaand all the species of small strongyles present. No efficacy was demonstrated against the tapewormAnoplocephala perfoliataor the horse botsGasterophilus intestinalisorG. nasalis