This study was undertaken to figure out the basic patterns of the frequency distribution of Ascaris lumbricoides in rural Koreans and to find out the effects of changing endemicity on the frequency distribution pattern. Total 853 cases in 6 areas were surveyed from November 1975 to February 1978. The mean ova positive rate was 33.1% and in the range from 23.9 to 66.7% by the surveyed unit. From the analyses of the collected data, the following results were obtained.

1. The overdispersion pattern of worm burden per case was mathematically fitted both to the theoretical values of the negative binomial and Poly-Eggenberger distributions. However, when "0" and lower burden cases were considered, the better fitness was observed in the negative binomial than in Polya-Eggenberger distribution.

2. Although the endemicity of Ascaris is changed, the relative frequency of the lower burden cases is rather constant.

3. The false negative rate of stool examination and unfertilized ova passers in the population were not changed significantly according to the endemicity. And the rate of unfertilized ova passers in infected cases was increased when the endemicity was lowered.

From the above results, it is concluded that the overdispersed frequency distribution pattern of Ascaris in an endiemic area is changed by the relatively rapid decrease of heavy burden cases and increase of non-infected cases according to the lowered endemicity due to the impact of the control.