The contribution of ammonium and phosphate contained in Noctiluca scintillans to the water-column nutrient pools were investigated in Harima Nada, the Seto Inland Sea, Japan. Ammonium nitrogen (NH4+-N) and phosphate (PO43--P) concentrations varied widely from undetectable to 36.5 μ M and to 1.6 μ M, respectively. Percentages of NH43--P contributed by Noctiluca were estimated based on population densities and the cellular nutrient contents. The NH4+-N and PO43--P contained in Noctiluca contributed to the water column NH4+-N and PO43--P pools ranging from 0 to 119% and to 80%, respectively. The estimated high percentages of both macro-nutrients were usually found in the middle of spring (April to June) where Noctiluca red tides were observable, indicating that during this period the nutrient regeneration by Noctiluca should not be ignored. The temporary enhancement of nutrients at the sea surface by Noctiluca during Spring season when low ambient nutrient concentration is characterized, effectively supports the growth of autotrophs and results in successive phytoplankton blooms.