The chemical and microbiological investigations of Kojima Bay were carried out for the purpose of clarifying the water pollution. The results and some discussions are summarizedd as follows. 1. Kojima Bay had been progressively polluted with organic compounds and inorganic nitorogen, but the situation of this bay has been improving for some years except transparency in the whole area and pollution by coli form group in the bay bottom. 2. The salinity of the surface water in Kojima Bay was conciderably low, half as much as that of the sea water. Then, Kojima Bay is in a suitable condition for the outbreak of red tides in a point of salinity. 3. The pollution load by the industrial and munincipal wast water drained into the bay is extremely too large for the volume of this bay. 4. In due considerations of the geographical condition and the pollution load, the waste water should be treated in closed system.