Genus Reticulitermes is widely distributed in the northern temperate zone. In Japan mainland there are two sympatric species, R. speratus and R. kamnonensis. Although these two species have similar ecological characteristics, their distribution patterns are markedly different: R. speratus is evenly distributed in Japan while R. kamnonensis is found only in restricted regions of Honshu (the Japan mainland) - e.g. in the westernmost part of Honshu. R. kamnonensis is considered to be an invasive alien species, but its range expansion speed is not so high as that of alien species should be. With a view to account for the difference in distribution characteristics, we studied the nestmate recognition of the two sympatric species, in termes of the agonistic behavior and trophallactic behavior. R. speratus showed strong agonistic behavior against different species and no trophallactic contact. However agonistic behavior against different colony of the same species was weak and trophallactic exchange of food was observed. On the other hand, R. kanmonensis showed strong agonistic behavior against different species and different colony of the same species and trophallaxis contact was absent. These results suggested that R. speratus has colony fusion as already known, although R. kanmonensis does not show colony fusion and strongly exclude non-nestmate individuals. It is suggested that the difference in distribution patterns of the two species is due to the difference in their nestmate recognition characteristics.