The relationship between the bloom of Coscinodiscus wailesii and water quality or meteorological factors was examined in Harima-nada between April 1995 and March 1998, by referring to the Meteorological Agency's data. Monthly changes in water quality factors and dominant phytoplankton showed that the bloom of C. wailesii occurred from November to February under conditions of high salinity, DIN (dissolved inorganic nitrogen）, DIP (dissolved inorganic phosphorus）, DSi (dissolved inorganic silicon）, and DIN×DIP, and low water temperature, DIN：DIP ratio (ca. 10≥） and TN×TP (total nitrogen×total phosphorus）: DIN×DIP ratio. The comparison of each average water quality factor among the dominant phytoplankton from autumn to winter showed that C. wailesii occurred in water with high DIN, DIP, DSi, and DIN×DIP, and a low TN×TP：DIN×DIP ratio, whereas Pseudonitzschia spp. occurred in water with a high TN×TP：DIN×DIP ratio compared to other dominant phytoplankton.