Microbial reduction of arsenate [As(V)] to arsenite [As(III)] and the subsequent effects on As mobilization in contaminated mine tailings were studied under transport conditions. Molecular analysis of bacterial populations and traditional isolation techniques were used in conjunction with column experiments designed to observe relationships among pH (limed vs unlimed treatments), redox potential (Pt electrode), and mobilization of As. Liming increased pH values from approximately 4 to 8, resulting in a 5-fold increase in total As eluted from sterile columns. Elution of As from limed columns was further enhanced by microbial activity. As(III) was the predominant As species eluted from oxic, nonsterile columns. Conversely, in sterile treatments, As(V) was the predominant valence state in column effluent. Denaturing gradient gel electrophoresis coupled with sequence and phylogenetic analysis of 16S rRNA gene segments revealed that liming of the mine tailings stimulated specific Caulobacter-, Sphingomonas-, and Rhizobium-like populations. Pure culture isolates of these bacteria demonstrated the ability to rapidly reduce As(V) in aerated serum bottles. An intracellular As detoxification pathway was implicated in the reduction of As- (V) by these isolates. These results indicate that microbial reduction of As(V) in As-contaminated soils may occur under aerobic conditions over relatively short time scales resulting in enhanced As mobilization.