Evidence is presented that bacterial communities enriched from aquifer sediments from Bangladesh mediate arsenic transformations. Bacteria from six different aquifers were cultured in the laboratory. Anoxic cultures from four out of six sediments tested exhibited high arsenate-reducing activity, while cultures from only one out of three sediments were shown to have high arsenic sulfide-oxidizing activity when incubated under oxic conditions. Our results suggest that sediment bacteria may transform arsenic under natural conditions and thereby play a pivotal role in the mobilization and immobilization of arsenic compounds in some arsenic-contaminated aquifers.