1. Studies were made on the uptake of phosphorus by the marine diatom Asterionella japonica, utilizing radioactive phosphorus as a tracer. 2. The higher phosphate concentrations in the solution were linearly related to higher contents of phosphate in the diatom cells. 3. A minimal cell concentration of phosphate for further growth was established for this species. 4. Two types of phosphate were found in the cells: the minimal content which was strongly bound and not removed by water washing of the diatoms and a labile phosphate which could be removed. Experiments on the release of phosphate conducted on diatoms containing only minimal amounts of it, indicated that only a small percentage of the phosphate was returned to solution as soluble phosphate after three weeks.