Restriction fragment length polymorphism (RFLP) was studied for six populations of the polytypic snail Neotricula aperta using seven different restriction enzymes. Samples of all three strains of N. aperta from north-eastern Thailand were examined, as well as topotypes from southern Laos and material from Kampuchea and central Laos. Genetic distances were estimated as Nei & Miller's (1990) distance ×100 (D). The samples were taken from the Mekong, Mul and Xé-Bang-Fai (XBF) rivers of the lower Mekong basin. The γ-strain of Kampuchea and southern Laos has been shown to act as intermediate host for Schistosoma mekongi. The least amount of genetic divergence was found where the α-strain was compared with other taxa (D, 2.1–3.7), and the largest values for comparisons involving samples of the XBF strain (D, 2.1–6.4). Large distances were apparent between the b-strain and all the γ-strain populations (D, 4.0–5.5). The β- and XBF-strain may represent a pair of sibling species with respect to N. aperta. The γ-strain population of north-eastern Thailand comprised two cryptic taxa which were relatively well diverged (D = 1.6). Only one of these cryptic taxa showed close affinity with other γ-strain taxa; this finding may be important in the control of Mekong schistosomiasis.

Variation was studied at 13 electrophoretically detected allozyme loci for samples of six populations of the polytypic snail Neotricula aperta; this was the first time that topotypic material from southern Laos and material from Kampuchea had been so examined. In addition, samples of all three strains of N. aperta from North East Thailand were examined and the findings compared with those of Staub et al. (1990). The samples were taken from the Mekong and Mul rivers of the Lower Mekong Basin. A small sample was also included from a γ-strain population in the Xé-Bang-Fai (XBF) river of central Laos. The γ-strain population of southern Laos and of Kampuchea has been shown to act as an intermediate host for Schistosoma mekongi.

In contrast to the work of Staub et al. (1990) significant multilocus genetic differences (Nei's D, DN = 0.6–1.2) were detected between the three strains from Thailand and no cryptic taxon, that included both α- and γ-strains could be demonstrated. However, there was agreement with the 1990 report in that the β-strain was divisible into two new taxa, with a reduction in heterozygote deficiency, although the genetic distance was not marked (DN= 0.12). Significant genetic distances were found between γ-strain snails from the type locality and β- and γ-strain samples from North East Thailand (DN = 2.0 and 1.5, respectively). On the basis of genetic distance measures, the Β-strain is probably a sibling species of N. aperta and the γ-strain of North East Thailand may also be a separate species. The population at XBF appears to be N. aperta s.s. The public health implications of the findings are discussed, particularly in the context of Mekong river water resource development.