Mebendazole was highly effective against the helminth parasite Trichinella spiralis in mice subjected to a 3-day course of treatment during the invasive and encystment phases of experimental trichinellosis. When treatment began either 2 or 4 weeks after the mice were inoculated with parasites, the number of larvae developing in the host musculature was greatly reduced by twice-daily oral administration of 3.125, 6.25, or 12.5 milligrams of mebendazole per kilogram of body weight.