ABSTRACTDiel vertical migration byHeterosigma akashiwo(Hada) Hada (Raphidophyceae) was monitored in a 1.5 in tall microcosm. Vertical stratification, with low salinity and low orthophosphate (Pi) concentration in the upper layer and high salinity and high Piconcentration in the lower layer, was simulated in the tank, analogous to summer stratification in the Seto Inland Sea. The phosphate metabolism of H.akashiwo during this vertical migration was studied using31P‐NMR spectroscopy. At night this species migrated to the lower phosphate‐rich layer and took up inorganic phosphate (Pi) which then was accumulated as polyphosphate (PPi) by an increase in the chain length of PPiDuring the daytime this species migrated to the phosphate‐depleted surface water and utilized the accumulated PPifor photophosphorylation by decreasing the chain length of PPiDuring the first night after the phosphorus was introduced to the previously impoverished waters, the cells took up inorganic phosphate, accumulating the new phosphorus nutrient internally as PiBut the cells did not convert Pito PPipresumably due to their lack of ATP. After the second day of the experiment, conversion of Pito PPiat night was much more rapid than on the first day, presumably due to increased ATP availability. Then the cycle continued, with uptake of Piand conversion to PPiat night at the bottom and its utilization during the day at the surface. These data suggest that the role of PPiin the metabolism of this species appears to be as a phosphate pool which regulates the level of Piand ATP in the cell. Diel vertical migration allows this red tide species to shuttle between the phosphate‐rich lower layer and the photic upper layer in stratified waters.31P‐NMR is shown to be a valuable tool in studying the phosphorus metabolism in migrating