The work investigated the efficiency of microbial fuel cells (MFCs) for the treatment of alkaline hexavalent chromium containing wastewater. When lactate was used as the metal chelator in alkaline (pH 8) abiotic cathodes, hexavalent chromium concentration dropped from 10 mg l−1 to undetectable levels within the first 45 h of operation. Power density produced in the pH 8 abiotic cathodes was up to 21.4 mW m−2, and in the pH 9 cathodes up to 2.4 mW m−2; these values were well comparable with other values found in the literature for biologically catalysed cathodes, even at lower pH values. When Shewanella oneidensis MR-1 was present in a hexavalent chromium reducing cathode at pH 8, current production contributed by 26 % to the total hexavalent chromium reduced during the 36 days of operation. On the other hand, when hexavalent chromium (10 mg l−1) was controllably added in the anode where S. oneidensis MR-1 was present, up to 73 % of current decreased immediately after every hexavalent chromium addition; this toxic effect remained even after hexavalent chromium was depleted in the anode and strongly indicates that the presence of hexavalent chromium in the anodes of MFCs must be avoided. Overall, our results indicate that alkaline hexavalent chromium wastewater can be effectively remediated in the cathodes of MFCs, provided that a metal chelator is present in the cathodes and that hexavalent chromium is not present in the anodes.