A summary of the systematics of the O. viverrini species complex. Depicted are the numbers of genetic groups (in yellow boxes) detected within each of the currently detected sibling species; the magnitude of fixed genetic differences between genetic groups; the levels of fixed genetic differences that define each species, and correlations of species and genetic groups with distinct wetlands after the application of the molecular genetic technique of allozyme electrophoresis. Fixed genetic differences occur when samples being compared do not have any alleles in common at the enzyme locus being examined. In the case of O. viverrini, 33 independent enzyme loci were examined [36]. The geographical distribution of O. viverrini includes Thailand, Laos, Cambodia, Vietnam and, possibly, Myanmar (no data are currently available).