Phosphate regeneration in vitro from algae decomposing in lake water was studied under two different content of dissolved oxygen in a room temperature-controlled at 25°C. The phosphate regeneration commenced including the decompositive process of algae. The rate of phosphate release highly depended on the content of dissolved oxygen in the first stage of the decompositive process. However, the absolute amount of phosphate liberation was independent of the content of dissolved oxygen after a certain period of decompositive process. Such trends were also obtained on the breakdown of particulate organic phosphorus About 79% of the total decomposed particulate organic phosphorus was converted into inorganic phosphorus under low concentration of dissolved oxygen and about 74% under high concentration, respectively.