Thirteen enzymes were detected electrophoretically in Thai pilid snails: acid phosphatase, aldehyde oxidase, esterase, glucose-6-phosphate dehydrogenase, glycerophosphate dehydrogenase, leucine aminopeptidase, malate dehydrogenase, phosphoglucomutase, phosphoglucose isomerase and xanthine dehydrogenase. Of these, 10 enzymes showed strong activity and genetic variability at 19 gene loci. The 36 heterozygous loci derived from these 10 enzymes were tested for G values. Only five deviated from Hardy-Weinberg expectations. According to genetic analyses, coupled with morphological characters, the genus Pila in Thailand has five species: P. ampullacea, P. angelica, P. gracilis, P. pesmei and P. polita. Of these species, P. ampullacea and P. polita are the most easily distinguished morphologically. The other three species, P. angelica, P. gracilis and P. pesmei, are all very similar morphologically and hence are more difficult to distinguish. The close relationships between these three species can also be seen from the allelic frequencies of the 12 polymorphic gene loci. The mean frequency of heterozygotes per locus varied from 0.035 +- 0.017 in Pila polita to 0.153 +-0.058 in P. gracilis. The percentage of polymorphic loci varied moderately in all species, ranging from 15.8 in P. ampullacea to 31.6 in P. gracilis and P. pesmei. The mean number of alleles per locus varied from 1.2 +- 0.1 in P. polita to 1.7 +- 0.2 in P. pesmei. Coefficients of genetic similarity (I) and genetic distance (D) were calculated using the analytical methods of Nei, Rogers and Wright. The average genetic identity (IN) between populations was highest (0.998) in Pila polita and was smallest (0.921) in P. gracilis. The phylogenetic trees derived from the unweighted pair-group method of clustering (UPGMA), using Nei’s I and modified Rogers’ distance, are similar topologically and the most consistent with the data in regard to the criterion of goodness-of-fit, which showed %SD=3.351 and 7.976 for Nei’s I and modified Rogers’ D, respectively. Shell morphology and reproductive anatomy support these phenograms. The phenogram constructed by the UPGMA method, and supported by morphology, shows that Pila gracilis and P. polita are closely related to each other, and are more distant from the other three species. Pila ampullacea, P. angelica and P. pesmei are themselves closely related, with Pila angelica being more closely related to P. pesmei than to P. ampullacea. Pila "scutata," which previously was considered to be a good species, is merely a non-banded form of P. gracilis.