The microorganisms Trichosporon cutaneum and Bacillus licheniformis were used to develop a microbial biochemical oxygen demand (BOD) sensor. It was found that T cutaneum gave a greater response to glucose, whereas B. licheniformis gave a better response to glutamic acid. Hence, co-immobilized T cutaneum and B. licheniformis were used to construct a glucose and glutamic acid sensor with improved sensitivity and dynamic range. A membrane loading of T cutaneum at 1.1x10(8) Cells ml(-1) cm(-2) and B. licheniformis at 2.2x10(3) cells ml(-1) cm(-2) gave the optimum result: a linear range up to 40 mg BOD l(-1) with a sensitivity of 5.84 nA mg(-1) BOD 1. The optimized BOD sensor showed operation stability for 58 intermittent batch measurements, with a standard deviation of 0.0362 and a variance of 0.131 nA. The response time of the co-immobilized microbial BOD sensor was within 5-10 min by steady-state measurement and the detection limit was 0.5 mg BOD l(-1). The BOD sensor was insensitive to pH in the range of pH 6.8-7.2.