About a hundred years ago, Henze discovered high levels of vanadium in the blood (coelomic) cells of an ascidian collected from the Bay of Naples. The intracellular vanadium concentration of some species in the family Ascidiidae can be as high as 350 mM, which is 107 times the concentration in seawater. Vanadium ions, thought to be present in the +5 oxidation state in seawater, are reduced to the +3 oxidation state via the +4 oxidation state and are stored in the vacuole of vanadocytes, the vanadium-containing cells, where high levels of protons and sulfate are also contained. To investigate this unusual phenomenon, we have isolated many proteins and genes that might be involved in the accumulation and reduction of vanadium. To date, more than five types of vanadium-binding protein, designated as Vanabin family, have been isolated from vanadocytes. In addition, four types of enzyme related to the pentose phosphate pathway that produces NADPH were revealed to be located in vanadocytes, and NADPH produced by the pentose phosphate pathway participates in the reduction of vanadium(V) to vanadium(IV). Vacuolar-type H +-ATPase (V-ATPase) maintains the low pH in the vacuole and is thought to provide the energy for vanadium accumulation. Using an immobilized metal-affinity chromatography (IMAC) and gene homology cloning, we have further obtained a Vanabin homologue in blood plasma, metalATPase, glutathione-S- transferase and SO4-2 transporter. Now, it becomes important to elucidate not only physiological roles of these proteins but also to resolve how these proteins share their roles to accumulate vanadium in ascidians.