In order to understand geomicrobial processes that mediate the mobility of As in natural settings, indigenous bacteria were isolated from As-laden geological materials under 10mM As(III) condition without supply of carbon source. These geological materials were soil from the black shale in Dukpyung area and sediment in the Dukeum mine area. The isolated bacteria were inoculated to 5mM As(III) solution, and variations in As species were monitored over time. The rate of microbial As(III) oxidation by the isolate from Dukpyung black shale soil was significantly slow, providing no clear distinction from chemical As(III) oxidation. In contrast, the rate of As(III) oxidation by the bacterium isolated from Dukeum mine sediment showed a remarkable increase when compared with chemical reaction. The results indicated that the oxidation of As(III) by indigenous chemoautotrophic bacteria may influence the speciation of As in natural geological settings and showed the potential for application to the effective As remedial processes in As-contaminated aquifer and subsurface.