The action of the ether of artemisinin (artemether) on Schistosoma mansoni in mice and hamsters experimentally infected with the LE strain was studied. In mice, the drug showed high schistosomicidal activity using a single intramuscular dose of 100 mg/kg/day. By the oral route, this dose showed a low activity. Mice treated with a single intramuscular dose of 200 mg/kg/day, and examined 15 days after treatment, presented 100% alteration of the oogram; when examined 45 days after treatment, the oogram was normal. With doses of 100 mg/kg/day, i.m., during 3 or 5 consecutive days, the death rate of mice was very high. Morphologic analysis of the worms collected by perfusion of mice treated with a single dose of 100 mg/kg/day, i.m., detected a marked decrease in the length of male and female worms, degenerative alterations in the parenchyma and in the reproductive system of the females, with the reduction of vitellinic material and in ovary volume; the intestinal contents presented a marked despigmentation. In the male worms significant alteration was not apparent by optical microscopy.