An "outbreak" of 22 cases of acute schistosomiasis from o new focus of S. mansoni in the periphery of Rio de Janeiro City is described. Fourteen children and 8 adults were infected in a short pericd of time while bathing in a waterfall in whose vicinity no snails were found. A small distance away, and on a higher level, however, the Biomphalaria tenagofila was found but the number of snails infected with cercariae of S. mansoni was very low. The authors believe that the explanation for this unusual "outbreak" could be that the snails or their cercariae had been washed down the waterfall in a recent heavy rainfall and thereby established the transmission of the disease. Out of the 22 cases of acute schistosomiasis described in this paper, 7 had severe clinical manifestations, 7 had moderate symptoms and signs and 8 had a mild, acute form of the disease. Fever was present in 17 cases, hepatomegaly in 14, "dry cough" in 13, astenia in 13, diarrhea in 12, loss of weight in 11, abdominal pain in 9, splenomegaly in 9, headache in 7 and nausea and vomiting in 3. The white blood cells count and eosinophilia were very high in the majority of cases. The eletrophoresis of serum protein showed a low albumin rate and an elevation of α 2 and γ globuin in some cases. The liver function test, the transaminases, chest X-ray, urine, blood and other tests zuere normal. The incubation period had an average of five weeks (15 to 45 days).