The effect of monobasic sodium phosphate on statolith synthesis in Aurelia metamorphosing in artificial sea water (ASW) and in low sulfate ASW was determined. Phosphate enhances statolith synthesis in organisms metamorphosing in ASW and restores statolith numbers to normal or above in organisms developing in low sulfate ASW. A small amount of sulfate must be present in the medium along with phosphate during the time period of statolith synthesis for normal statolith formation. Apparently, neither sulfate nor phosphate is stored in the organisms during early strobilation for later use in statolith synthesis because pre-treatment of either ion in early strobilation followed by treatment with the other ion does not result in statolith formation. Calcifying vesicles of the rhopalia of organisms from low sulfate ASW are normal in number, acid phosphatase activity, and in ability to initiate mineralization (by forming minute statoliths). While phosphate is not incorporated directly into the statoliths, it contributes to an efficient uptake of calcium and sulfur into the cells and/or calcifying vesicles, stimulating growth of calcium sulfate dihydrate statoliths. The high efficiency of the phosphate effect in enhancing statolith synthesis intracellularly demonstrates that phosphate acts at the cellular level in the jellyfish calcification process and emphasizes that phosphate probably plays multiple roles in the calcification of higher organisms.