Abstract: Field observation on the population fluctuation of Oncomelana snail at low density was carried out in the ditches and rivers in Wuxi,Ludong and Lugao counties (or city) from 1G87 to 1990.The results showed that the environment of the observed places is an important factor in the population fluctuation of the snails at low density.When the environment is appropriate,even one pair of the snails may multiply to a large populationResults of successive observation for 3 years on 1,5 and 10 pairs of snails within nylon cages in ditches showed that the density of the snails increased by 171.5,69.5 and 28.4 folds respectively and the population of the snails increased by 354,135 and 75 folds respectively.The population in 2 of the observed ditches with one pair of snails each even multiply by 543 and 426.5 folds.While in natural field condition,the population fluctuation of residual snails varied in different places.In one ditch it multiptied by 34.7 folds,but in another river the population almost unchanged.No natural elimination of snails could be found in all observed places.It was suggested that the snail surveillance should be conducted persistently and any snail once be found,it shou'd be timely eradicated.