Abstract: The behavior changing of extracellular polysaccharide in leaching bacteria was studied under the different concentrations of As(Ⅲ) and As(Ⅴ) stress，in order to investigate the relationship between extracellular polysaccharide in leaching bacteria，concentration and valence of arsenic ion，and increase indexes of leaching bacteria，so as to find out biological strategies of leaching bacteria against arsenic ion.The pH，Eh，Fe²⁺ concentration，the number of bacteria and the content of extracellular polysaccharide were measured to analysis the correlation and the linear relationship between them.The results show that under As(Ⅲ) stress，the production of extracellular polysaccharide in the bacterial liquid increases over 34%，As(Ⅲ) stress effect is stronger than As(Ⅴ)，and the extracellular polysaccharide content is linear correlation with Eh，Fe²⁺ concentration and the number of bacteria at level of 63.4%，61.6% and 71.4% respectively.The results also indicate that leaching bacteria can regulate and control the growth indexes of the bacterial liquid to adjust the amount of extracellular polysaccharide，thus to resist arsenic ion stress.

Key words: Bacteria metallurgy, Arsenic ores, Arsenic ion stress, Extracellular polysaccharide