1.Each cells homogenized from Undaria were reacted in reaction micture to persue the phosphate metabolism in Undaria cell. Aliquots of the cells were taken out at the begin-ning and at intervals during the reaction, and analyzed for the content of total-P in various fractions of the cell constituents. 2.The P-contents in fraction of polyphosphate "B" decreased remarkably, while that in fraction of RNA polyphosphate "C" showed slow increase. 3.As well as in Chlorella cells, inorganic phosphates in DNA-P, protein-P, and lipid-P were transferred from polyphosphate, RNA-P turnovered from inorganic phosphate that is in cytoplasm, and RNA polyphosphate complex from polyphosphate, and it was suggested that inorganic phosphates in polyphosphate "B" could transformed into polyphosphate "A" & "C", and polyphosphate "C" into polyphosphate "A".