The monthly cercaria shedding of 90 artificially infected Oncomelania snails was observed under a field condition in mountainous region, Shitoudi Village, Weishan County, Yunnan Province. The results showed that 20% of teh snails shed cercaria monthly, 42.2% could shed irregularly and 37.8% stopped releasing cercariae after several times of shedding. Final direction of the snails showed that none cercaria or sporocyst could be found in part of the snails. A total of 304 naturally infected snails were observed individually every month. Samples were taken each month from the snails which did not shed cercaria, then dissected and examined. The negative conversion rate was calculated in the second, third, fourth, fifth and sixth month after the first shedding, which were 36.1, 50.0, 41.0, 39.8 and 2.6% respectively.