A formulation for treating and preventing roundworm infestations in horses, dogs, cats, and other animals is provided. This formulation includes an agent effective against nematodes, wherein the agent is pyrantel, morantel, any salt thereof, or any combination thereof. Preferably, the formulation additionally includes an agent effective against nematodes and obligate parasitic flies, an edible non-aqueous liquid and a thickening agent. The formulation of the present invention allows for a high concentration of the agent effective against nematodes and for the treatment of roundworms, particularly ascarids, that have become resistant to at least one agent typically effective against nematodes.