Field measurements of water quality at the mouth of Tokyo Bay had been carried out once a month from July 1981 to June 1984. The measuring water quality items are water temperature, salinity, dissolved oxygen, suspended solid, transparency, illuminiation, water color, color name, turbidity, and chemical oxygen demand. In this note, the data from November 1982 to June 1984 were analyzed and the sea condition at the mouth of Tokyo Bay in summer and in winter was investigated.
　The relations among water quality itmes, closely related to the water contamination, were investigated by calculating the correlation coefficients.
　The general characteristics of water quality were found by the principal component analysis and the water quality items were classified by the cluster analysis.
　It was observed that in the sea, where a red tide was occurred, the transparency and salinity were significantly lower and the temperature, dissolved oxygen and illumination coefficient were significantly higher than in the surrounding sea where a red tide was not occurred.