Deep seawater (DSW) samples were collected from 320 m depth layer at an offshore station close to the pipe end of DSW-intake facility of Muroto City in Kochi, and seasonal fluctuations of the water quality of DSW were investigated. Concentrations of dissolved inorganic nitrogen (DIN) and phosphate in DSW varied significantly in the range of 6-26 (average 15.7) μ M and 0.7-1.8 (average 1.2), μM, respectively. High concentrations of these inorganic nutrients apparently corresponded to upwelling seasons. Algal growth potential (AGP) of DSW for a diatom, Skeletonema costatum, also varied from season to season, and high AGP values were obtained when the concentrations of DIN in DSW increased. These results indicate that water quality of DSW fluctuates, to relatively large extent, probably due to possible upwelling events, and seasonal changes in AGP of DSW were resulted from DIN content in DSW.