Arsenic accumulation by hyperaccumulator, Pteris vittata, when exposed to arsenite in hydroponics is comparable to that when exposed to arsenate while arsenite uptake is considerably slower than arsenate uptake. Arsenite species around root and that in the incubating solution were analyzed to investigate arsenite oxidation in hydroponics. XANES analysis of the fern roots showed that arsenate predominated in all parts tested, root tip, middle, and root top, even an hour after the addition of arsenite, suggesting arsenite oxidation in whole rhizosphere. Although arsenite in the rhizosphere is rapidly oxidized to arsenate within an hour, arsenate was not observed in the bulk solution and was gradually increased during 18 days-incubation, supporting the involvement of rhizosphere in arsenite oxidation. Arsenite speciation of medium during arsenite uptake experiment showed that in 7 days, the most arsenite was oxidized and remained as arsenate with a fern conditioned with 1mM phosphate (Pi), whereas the most arsenite was decreased with a fern conditioned with 10 M Pi, suggesting arsenite oxidation and subsequent phosphorus-starvation-dependent uptake of the arsenate.