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.