Lake Abashiri and Lake Saroma are located along the coast of Ohotuku Sea, east part of Hokkaido, northern Japan. The salinity of lake water is approximately 5 permil at Lake Abashiri, and 30 permil at Lake Saroma. We took 23 samples of surface sediments and two bore hole samples from Lake Abashiri, and 26 samples of surface sediments and two bore hole samples from Lake Saroma for the analysis of diatom assemblages.
　Cyclotella caspia, a planktonic diatom, is dominant at Lake Abashiri, and percentage of it among diatom valves from surface sediment is above 90% at most of sampling site. The diatom assemblages from bore hole samples show that Lake Abashiri was a fresh water lake, because Aulacoseira granulata, Aulacoseira ambigua and Stephanodiscus dubius, fresh water Planktonic diatoms, were dominant from the lower part of the bore hole samples. We got no dating data from the samples, yet.
　Thalassiosira spp. And Thalassionema nitzschioides, planktonic diatoms, are dominant at central part of Lake Saroma. On the other hand, Cocconeis scutellum, a benthic diatom. Is dominan,t at the sites near the lake coast. The diatom assemblages and data of sedimentary ages (210Pb method) from bore hole samples show that the salinity of Lake Saroma had increased in 1929.