Capability of utilization and excretion of dissolved organic phosphorus (DOP) by the toxic dinoflagellate Alexandrium tamarense (Hiroshima Bay strain) was investigated with both batch and semi-continuous cultures using natural aged-seawater from Hiroshima Bay. In the batch cultures, significant net DOP uptake was detected in phosphate concentrations below 2μM, while net DOP excretion was observed above this concentration. The semi-continuous culture showed that A. tamarense excretes DOP at high dilution rates. At these rates, this organism grows fast by utilizing sufficient phosphate in the medium. These experiments, although preliminary, suggest that A. tamareme (Hiroshima Bay strain) can utilize DOP of natural seawaters under a condition of low phosphate concentration and excretes DOP under a condition of high phosphate concentration.