Enterobacter cloacae CYS-25 strain was isolated from a chromate plant. This bacte-rium was capable of resisting high hexavalent chromium concentration and reducing Cr(Ⅵ) under aerobic condition. CrO42- stimulated the increase of bacterial size and production of compact convex paths containing chromium on the bacterial surface. The increase of bacterial size was caused by integrative growth but not extracellular polymeric substance hyperplasia. IR and SDS-PAGE analyses showed the extraeellular polymeric substance (EPS) components were mainly proteins and had no obvious changes whether the strains were induced by Cr(Ⅵ) or not. The EPS was amorphous and contained trivalent chromium. Under CrO42- growth condition, the extracellular substance of Enterobacter cloacae CYS-25 strains and Cr(Ⅵ) had redox reaction. The products were Cr3+-protein complexes which formed a piece of compact convex paths on the surface of bacteria and prevented Cr(Ⅵ) from entering into cells.