BACKGROUND: Chromium removal potential of the cyanobacterium Anacystis nidulans and its chromium resistant strain Cr(r)18 has been optimized. Optimized parameters include biomass load, pH, temperature and dilution rate of the bioreactor. RESULTS: Results show that chromium resistant strain has high EC50 dose for chromium compared to wild strain. Chromium removal potential of both strains is strongly influenced by various factors. Optimized conditions in batch system included pH 6.5, temperature 28 degrees C, biomass load 150 mu g protein mL(-1) for 30 mu mol L-1 Cr6+ solution. In continuous flow bioreactor at optimum pH (6.5) and temperature (28 degrees C) at a fixed biomass of 10 g protein and 30 mu mol L-1 Cr6+, metal removal efficiency varied with dilution rate. For A. nidulans continuous flow bioreactor, optimum dilution rate was 0.076 h(-1) (64.6 per cent metal removal) while for Cr(r)18 it was 0.152h(-1) (85.8 per cent metal removal). Operative time of the Cr(r)18 bioreactor was also more (85 h) compared to A. nidulans bioreactor (45 h). CONCLUSION: Under optimized conditions resistant strain Cr(r)I8 removed more Cr6+ compared to A. nidulans and thus has the potential to be exploited for metal removal from industrial effluents at large scale. (c) 2007 Society of Chemical Industry