This paper describes the characteristics of water quality at a red-tide in coastal region using the multivariate analysis. The data used here were obtained at the side of the Nishinomiya Breakwater in the Osaka Bay to develop the automatic measurement system of water quality.
　The automatic measurements of water quality were carried out for 3 durations. The measurements were conducted for 15 day in 1983, for 30 days in 1984, and for 40 days in 1985, respectively, at intervals of one hour. Water quality itemsty, pH, DO, SS, COD, chlorophyl-a, nitrate, ammonia, phosphate, total phosphorus, and silicate.
　With the time series data, the author investigaed the characteristic features of variations of these water quality items by multivariate analysis such as principal component analysis, multiple regression analysis, cluster analysis, correlation analysis, etc.
　From the results of the study, it is found that the synthetic characteristics of water quality is expressed by the several items related to red tide plankton, and the most suitable index or red-tide is the concentration of chlorophyl-a.