Several layers densely containing the bivalve mollusk Potamocorbula sp. are observed in the sediment core from Lake Shinji．The characteristics of the layers are as scarcely having any fossils except Potamocorbula sp.，most of the shells being juvenile stage of 4 to 5 mm in length and horizons of them overlapping with diatom barren zone．On the other hand，P. amurensis invaded San Fmncisco Bay，Califomia，USA，in 1986 from Asia as veliger larvae in the seawater ballast of cargo vessels and spread throughout the estuary within 2 years．Making reference to the continuous record of the appearance and spread of P.amurensis in San Francisco Bay carried out by the project team of U.S. Geological Survey，the meaning of Potamocorbula layers in the core from Lake Shinji was discussed．
1）The appearance of Potamocorbula sp. in the core from Lake Shinji was related to the appearance of the enclosed low-saline water area of 7800 to 7900 years ago，which was caused by the climate of high precipitation before and after the K-Ah tephra horizon of 7300 Cal Y.B.P.
2）The reason why diatom becomes barren in the beds of Potamocorbula sp. is supposed to be the remarkable effect of filtration of the clam．
3）The swarm occurrence of Potamocorbula sp. in the layers is considered to be a condensation by the repeated mass production of dead shells by the seasonal appearance of anoxic bottom condition and the small amount of sediment supply to the bottom．
In Japan，the habitat of Potamocorbula spp．overlaps with that of Corbicula japonica，one of the most important clams in fishery．lf Potamocorbula spp. invaded Lake Shinji where the highest catch of C.japonica has been recorded in Japan，it can readily be imagined that the economy of this area as well as the ecology in the lake must suffer a great deal of damage．It is fully significant to clarify the mechanism of invasion of Potamocorbula sp．in the past sediments for the prevention and estimation of damage．