The kinetics of chromium(VI) reduction by Pseudomonas aeruginosa (P. aeruginosa) and Escherichia coli (E. coli) was studied under both pure and mixed cultures. Initially, the study of kinetics was performed in pure culture. It was observed that the growth of the two bacteria was both inhibited in the presence of chromium(VI). The maximum specific growth rate (mu(m)) of P. aeruginosa decreased from 2.3942 h(-1) (without Cr(VI)) to 1.8551 h(-1) (with Cr(VI)). Under the mixed culture, the growth of E. coli was inhibited by P. aeruginosa. The maximum specific growth rate (mu(m)) of E. coli decreased form 0.871 h(-1) (in pure culture) to 0.153 h(-1) (in mixed culture). When the concentration of each bacterium was 4.5 x 10(8) cells ml(-1), the half-velocity reduction rate constant (K-C) and the maximum specific reduction rate constant (v(max)) of chromium(VI) were 80.05 mg chromium(VI) l(-1) and 3.674 mg chromium(VI) cells(-1) h(-1), respectively. The results showed that the simulation appeared in good agreement with the experimental data, supporting the series of mathematical models represented the bacteria growth and chromium(VI) reduction in both pure and mixed cultures usefully.