The rates at which phosphorus is (1) assimilated by phytoplankton, (2) released into solution from dead cells, and (3) regenerated to inorganic state, were measured in outdoor concrete tanks containing sea water fertilized with inorganic phosphate and nitrate. Bottles wrapped in black cloth and bottles exposed to the light were filled with the tank water and suspended in the tanks; the three rates in the P cycle were calculated from the observed changes in the concentrations of inorganic and particulate P in the bottles. The maximum recorded rates were: assimilation = 0.36 µg-at. P/L of sea water in the tanks/day; solution = 0.38 µg-at./L/day; regeneration = 0.13 µg-at./L/day. In 18 series of measurements, during which phytoplankton increases predominated over decreases, the following relations were observed between the rates of phosphorus transformations and the size and absolute change of size of the phytoplankton populations: (1) The rate of phosphorus assimilation was significantly correlated with the size of the phytoplankton standing crop and showed an even stronger dependence upon the increment of growth. (2) The rate at which particulate organic phosphorus was released into solution was intimately related to the size of the standing crop and was independent of th~ change in population size (which in most cases was an increase). (3) The regeneration of dissolved inorganic phosphate from dissolved organic phosphorus depended upon the phytoplankton standing crop and showed no relation to the change in population size. If attached algae were allowed to grow on the sides and bottom of the tanks, in less than one month they removed three-fourths of the added phosphate from the waterphytoplankton system. Where attached algae were largely prevented from growing, four-fifths of the phosphorus originally present was detected in the water at the end of four weeks