This paper is concerned with medium optimization for the production of bacteriorhodopsin (BR) by Halobacterium salinarium PTCC 1685 by one-factor-at-a-time and Taguchi's array methods. The one-factor-at-a-time method was used to study the effects of various carbon and nitrogen sources, temperature and initial pH on bacteriorhodopsin production. Among these variables, corn steep powder, universal peptone and yeast extract were the most suitable sources, respectively. The optimal temperature and initial pH for bacteriorhodopsin production were 38 degrees C and 7.5, respectively. Subsequently, the concentration of corn steep powder and other medium components were optimized using Taguchi's method. The results showed that corn steep powder, glycerol and meat extract had significant effect on bacteriorhodopsin production by H. salinarium PTCC 1685. The optimal concentrations of nutritional components for improved bacteriorhodopsin production were determined as 10 g l(-1) meat extract, 3.75 g l(-1) casamino acids, 10 g l(-1) glycerol, 50 g l(-1) corn steep powder, 1 g l(-1) Na(3)-citrate, 6 g l(-1) KCl, 0.2 g l(-1) FeSO(4)center dot 7H(2)O, 0.2 g l(-1) MnSO(4)center dot 7H(2)O and 10 g l(-1) MgSO(4)center dot 7H(2)O. The maximum bacteriorhodopsin production in shake flask culture was 191.7 mg l(-1) about 2.85-fold higher than those obtained with the basal medium. Under optimal culture conditions, the maximum bacteriorhodopsin production in a 15-l stirred-tank bioreactor indicated 234.6 mg l(-1) about 3.49-fold higher than the basal medium. (C) 2008 Elsevier Ltd. All rights reserved.