A series of simple experimental systems combining seven different As-minerals with natural Fe-rich underground water (pH 7) was established in search of clues to arsenic bioremediation. After one year of aging, arsenic-tolerant microorganisms were detected by means of an epifluorescence microscope, SEM-EDX and TEM. Direct observations revealed that a mutual relationship between bacterial colonies and As-minerals occurred in the biofilm. The most arsenic-tolerant microorganism was found on the surface of realgar (AsS) and the second one was found on the surface of arsenolite. A symbiotic relationship between free living and adhering bacillus, coccus, and filamentous types of bacteria occurred under conditions of pH 8, Eh 200～400 mV, 25～28℃, and high concentration of dissolved As after one year of aging. These microorganisms survived and proliferated in the As-mineral-water systems without any kind of supplementary nutrients. There were two methods of staining used in this study: DAPI and CFDA stains. The DAPI stain method, in which DNA or RNA fluorescences blue under ultra violet light (365 nm), indicated the total bacterial abundance with in the colony. The CFDA stain method indicated enzymatically active bacteria on the mineral surface. The significant roles of symbiotic arsenic tolerant bacteria in the system were enhanced by their adaptation to conditions of pH 7～8 and high concentration of dissolved As ion. SEM images illustrated very high cell densities on the surface of realgar. Colonization occurred on AsS surfaces in natural solutions during a one-year culture period. These results might be important for solutions at circum-neutral pH in which abundant cell-mineral surfaces are able to adsorb metals from solution. The results might also indicate some of the effective factors for arsenic detoxification in non-marine or brackish waters.