Acid sulfate soil is the common name given to soils containing iron sulfides (pyrites). When the pyrites are exposed to air and oxidized to sulfuric acid, both soil and water show a lower pH (actual acid sulfate soils, AASS); and serious destruction of the environment ensues. Although it is possible to maintain farming by liming together with precise control of irrigation, such measures are too expensive for developing countries. It is, therefore, necessary to develop more economical measures. One possible measure is bioremediation using organisms adapted to AASS. To develop effective bioremediation methods for AASS, we analyzed Ludwigia adscendens, a plant adapted to AASS, which was collected at a highly acidic swamp (pH 3.4) in an AASS area of the Hoa An research station of Can Tho University, Viet Nam. In acidic culture solutions L. adscendens alkalinized the culture solutions by increasing ammonia concentrations of the culture solutions to the level necessary for the alkalinization; whereas in a neutral culture solutions no or little alkalinization was observed, suggesting that alkalinization is an adaptive response of L. adscendens to acidic conditions. On acidic agar culture plates ammonia was detected around the roots of L. adscendens but not around the stems and leaves. These data suggest that the increase in the ammonia concentration of the culture solutions was an adaptive response of L. adscendens to the acidic conditions. The knowledge obtained from such studies should greatly help the development of new bioremediation methods for AASS. TA was supported by funding from the “Center of Excellence in 21 Century Projects” of the Ministry of Education, Culture, Sports, Science and Technology.