Abstract: The annual incidence is a ratio of positives having been converted from negatives during one year. Children at 9 primary schools at Dagami Area, Leyte in the Philip-.pines were followed up from 1974 to 1983 and the incidence rates of schistosomiasis japonica were calculated by the results of fecal examination and COP reactions amngo 1,800 children enrolled in these schools. The survey data were stored, processed and calculated by computer; The observed annual incidence rates were 22.2, 24.2, 26.9, 9.6, 28.4, 8.4, not examined and 6.8% yearly from 1975/76 school year (SY). on. As to control measure, environmental modifications with drainage by. channelling ditches In ;the rice field at the most depressed area of 50 ha in the middle of project area at 5 km radius had been conducted from Dec. 1974 to Mar. 1979. After Feb. 1979, mass chemotherapy with praziguantel has been given to egg positives of children and adults in this area. The cause of abrupt decrease of incidence to 9.6% in SY 1978/79 was thought to be due to less rain fall in the rainy season from Nov. 1977 to Jan. 1978. In the period of environmental modification, the incidence was affected little and was greatly lowered after mass chemotherapy. It was concluded that the small scale environmental modification affected little on reducing danger of infection even to the close area, although it was effective in many places in the world when it was designed and conducted at a large scale.