Sanaru-ko is a highly eutrophied brackish lake, 0.6km. in width and 22km. in length, situated near the City Hamamatu in the prefecture Siduoka. Almost all the quantities of Hypomesus olidus (Pall.), Konosirus punctatus (T. & S.) and other fishes died in the short period, from the evening of 30th, to the morning of 31st., August 1933. In this paper the causes of this sudden occurrence were investigated by us in view of chemical, biological and meteorological conditions. Since the middle part of the month of August a remarkable outburst of water-bloom, mainly consisting of Clathrocystis aeruginosa Henfrey had been occurred and before a few days of this unusual event the phytoplankton died and sunk down into the bottom layer. As a result of putrefaction of the organisms the O₂-consumption and H₂S-production (3mg./L) were made and moreover the supply of O₂ was very poor by the prevalence of the calm weather.