This article reports main results of investigations on red tide in western waters of Xiamen Bay from Jan. to Dec. 1987, and elaborates on the relationship between the change of phytoplanktonic number and main environmental factors there. The cause and the method for judging and monitoring of red tide are discussed. This article reports main results of investigations on red tide in western waters of Xiamen Bay from Jan. to Dec. 1987, and elaborates on the relationship between the change of phytoplanktonic number and main environmental factors there. The cause and the method for judging and monitoring of red tide are discussed.