a species-specific anthelmintic against Haemonchus contortus, most useful in areas where benzimidazole resistance has developed in the worm population and where frequent treatment is necessary. It also has the advantage that it is used at a single dose level for sheep of all sizes. It is an organophosphate compound but is safe at the recommended dose rates. Higher dose rates are of moderate efficiency against most intestinal nematodes of sheep, but may be toxic. Called also phthalophos.