To observe the effect of various interval mass treatment on Ascaris ova and worm positive rates and worm burden per population, 2-month, 4-month, 6-month and 12-month interval schemes were designed and applied. All of inhabitants in nine villages of Hwasung Gun, Kyunggi Do, Korea were treated from April 1977 to May 1980.

Each village consisted of 100~140 people, however, there had been 20~40 drop-outs in every treatment. Pyrantel pamoate was used in dose of 10 mg/kg. Evaluation of schemes was made by examinations both for eggs and for expelled worms. The drop-outs in former treatment was included because they were few in munber. The reults obtained were summerized as follows:

1. The pre-treatment infection status of A. lumbricoides was not significantly different between Groups; 32.5-42.2% and 33.8-46.2% in egg and worm positive rates respectively. The mean worm burdens were in range, 1.6~4.2 per infected and 0.61~1.42 per population.

2. Twelve-month interval treatment was by no means meaningful to be adopted as a strategy of Ascaris control because egg and worm positive rates and worm burden were returned to pretreatment level.

3. The shorter the treatment interval was, the lower the egg/worm positive rates and worm burden were. By repeating biannual, triannual and six-times-in-a-year treatments, the indices of prevalence showed the tendency of further lowering during later 2 years of follow-up.

If the mass chemotherapy is adopted as a method of controlling ascariasis in a community, it should be repeated more than two times in a year to expect the gradual lowering of reinfection.