An electrochemical 'Lab-on-a-chip' for water toxicity detection is presented. This miniaturized device containing an array of nano liter electrochemical cells, which integrates bacteria and can emulate physiological reactions in response to different chemicals. Bacteria, which have been genetically engineered to respond to environmental stress, act as a sensor element and trigger a sequence of processes, which leads to generation of electrical current. The silicon chip contains an array of nano-volume electrochemical cells that house the bacteria, connected to a sensing and data analysis unit. Each of the electrochemical cells in the array can be monitored independently and simultaneously with the others. A measurable current signal, well above the noise level, was produced within less than 10 minutes of exposure to representative toxicants, This miniature device provides high throughput rapid and sensitive real-time detection of acute toxicity in water.