Ascidians are sessile marine animals known to accumulate high levels of vanadium selectively in vanadium-containing blood cells (vanadocytes). Almost all the vanadium accumulated in the vacuoles of vanadocytes is reduced to the 3 oxidation state via the 4 oxidation state, although vanadium is dissolved in the 5 oxidation state in sea water. Some of the reducing agents that participate in the reduction have been proposed. By chemical study, vanadium in the 5 oxidation state was reported to be reduced to the 4 oxidation state in the presence of NADPH. The present study revealed the existence of glucose-6-phosphodehydrogenase (G6PDH), the first enzyme to produce NADPH in the pentose phosphate pathway, in vanadocytes of a vanadium-rich ascidian. The results suggested that G6PDH conjugates the reduction of vanadium from the 5 through to the 4 oxidation state in vanadocytes of ascidians.