There were 2,471 school children suffering from enterobiasis in six primary schools of Tainan city. The infected children in five primary schools, Po-Ai, Yung-Hua, Fu-Hsiao, Pao-Jeng and Hsin-Nan, were treated with one single dosage of 100 mg mebendazole. In Shih-Men primary school, the sixth, the students were given placebos as a control group. Negative conversion rates of infected children were examined after three weeks of chemotherapy and school children in the six primary schools were surveyed for enterobiasis two months after chemotherapy to obtain infection rates. The method of examination was two consecutive-day adhesive cellophane perianal swabs. With the purpose of evaluating the efficiency of treatment, positive reduction rates were used and calculated according to the infection rates of school children gained before and after chemotherapy. Those rates in Pao-Jeng, Fu-Hsiao, Yung-Hua, Po-Ai and Hsin-Nan were 62.1%, 47.8%, 41.8%, 37.1% and 3.3%, respectively, and in Shih-Men 3.9%. Judging the data obtained from each grade or each class of schools showed that the efficacy of chemotherapy in reducing the rate of infection was variable. Hence, one single dose of mebendazole and education on personal hygiene were not sufficient to reduce the prevalence of enterobiasis in primary schools. This was because the cycle of E. vermicularis was relatively short, cutting out the routes of transmission was very difficult, and the factors involved were very complex. Overall, though the positive reduction rate presented in Hsin-Nan primary school showed nearly no success in reducing the infection, the other four schools showed valuable rates. Whether continue a treatment, of about three-months in duration, can inhibit the prevalence of enterobiasis among children in primary schools or not will need further study.