Epidemiologic studies on schistosomiasis japonica were carried out in the Obitsu River basin of central Boso Peninsula, a newly found habitat of Oncomelania snails in Japan. Forty-five (4.1%) of 1,102 schoolchildren and 64 (10.4%) of 618 junior high school students were positive for the skin test with veronal buffered saline-extracted Schistosoma japonicum antigen. None was positive by the enzyme-linked immunosorbent assay, complement fixation test, double diffusion test in agar, and circumoval precipitin test. About one fifth of 356 adult inhabitants showed positive reactions in the enzyme-linked immunosorbent assay and complement fixation test as well as in the skin test, although only 25 individuals (7%) were positive for the circumoval precipitin test. All of 70 individuals who had shown positive reactions in at least two of the immunological tests were negative for a series of 5 stool examinations with the AMS III method. Release of S. japonicum cercariae was not observed in 15,000 O. hupensis nosophora collected in this area. Repeated examinations by dipping mice into irrigation ditches were negative. These observations, with the fact that all of 15 patients reported from nearby hospitals to have schistosome eggs in their tissues were over 45 years old, suggest that schistosomiasis has become a disease of the past in this area.