In order to estimate the genetic differentiation of natural populations of the snake-head fish in Thailand, 12 enzymes were examined in 13 populations collected from the north, the central, and the northeast region. Seventeen enzyme loci were detected, and polymorphism at 6 loci was revealed. The characteristic frequencies of alleles were regionally observed at IDHP-2* and MPI* locus. The genetic distances among populations within region were smaller than among populations of 3 regions. The average genetic distance among populations between the north and the northeast region and between the central and the northeast region were larger than that between the north and the central region clearly. The populations could be divided into 2 groups of the north-central and the northeast population on the basis of the dendrogram drawn by genetic distances. The genetic groups coincided with the geographical groups divided by 2 large river systems in Thailand.
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