This study examined the relative importance of environmental factors and geographic isolation on the distribution of apple snails in Hong Kong two decades after their invasion from South America. A survey of 61 sites was conducted to collect apple snails and measure 18 environmental parameters known to influence mollusk distribution. Identification of specimens collected in our study was aided by analysis of DNA sequences, and all apple snails collected in Hong Kong were identified as Pomacea canaliculata. Since its initial introduction in the early 1980s, the distribution of this invasive snail has only expanded slightly. Principal component analysis showed that the environmental characteristics of the study sites varied with habitat. Streams were quite homogenous in chemical characteristics and contained little dissolved minerals, whereas ponds, abandoned wet farmlands and drainage channels all showed great variations in nutrient loading. Discriminant function analysis (DFA) revealed that the inhabited sites typically had high levels of phosphate and alkalinity, but the snail was also occasionally found in streams where dissolved ion concentrations and nutrient levels were low. Most of the inhabitable wetlands in New Territories have already been occupied by P. canaliculata. Because of its unsuitable hydrology, Hong Kong Island remains uninhabited by this species. Lantau Island has habitable sites for this species, and thus is susceptible for invasion in the future.