The liver fluke is an important human parasitic disease in Southeast Asia. Its risk is related with the land utilization pattern along rivers, because its intermediate hosts, a Bithynia snail and fishes, dwell in paddy regions. Recently, the middle Mekong area confront drastic modernization of the irrigation system, but information on the ecology of the snail is too limited to access the effect of such changes on its distribution. To examine factors affecting its distribution and age structure, field surveys were done in Lahanam, Lao PDR in Feb-Nov 2011. GLM showed that both abundance and biomass of Bithynia snails are higher in habitats with mud bottom, high turbidity, low conductivity and no current velocity, such as paddy fields and clay-bottom channels. The effect of modernization of irrigation system was positive in paddy fields (double-cropping), but negative in channels (concrete bank protection). More Bithynia snails were observed in the dry season than in the rainy season. Size composition became bimodal twice a year in double-cropping paddy fields and clay-bottom channels, whereas it remains unimodal throughout the year in ponds. Further modernization of irrigation system would expand its distribution, double its production rate, and possibly enhance the risk of the liver fluke in paddy fields.