Continuous biohydrogen production with calcium supplementation at low hydraulic retention time (HRT) in a continuous stirred tank reactor (CSTR) was studied to maximize the hydrogen productivity of anaerobic mixed cultures. After stable operations at HRT of 8-4 h, the bioreactor became unstable when the HRT was lowered to 2 h. Supplementation of 100 mg/L calcium at HRT 2 h improved the operation stability through enhancement of cell retention with almost two-fold increase in cell density than that without calcium addition. Hydrogen production rate and hydrogen yield reached 24.5 L/d/L and 3.74 mol H(2)/mol sucrose, respectively, both of which were the highest values our group have ever achieved. The results showed that calcium supplementation can be an effective way to improve the performance of CSTR at low HRT. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.