We examined the genetic variation within the species, the patterns of genetic diversty between populations, thermostability variations of enzymes and temperature tolerances of Corbicula japonica from the two main rivers in Korea. Starch gel electrophoresis was used to examine the genetic variation of 22 loci. Heating experiments of electrophoresis under the condition of 40±5° for 15±5 min disclosed thermostability differences, called heat-sensitive and heat-resistant types, within each electrophoretic allozyme. Genetic diversity at the natural species level was high (77.3%), whereas the extent of heat-treat groups was relatively low (52.6%). The genetic diversity trends to decrease from the source of two main rivers (the Sumjin River and the Nam River) to the mouths. Based on the data available such as considerable high genetic diversity compared with a mean value of C. japonica species, It is recommended that several populations of the species in Korea should be preserved.