Microbial whole-cell bioreporters have been developed for environmental monitoring of arsenic contamination. Despite the great interest in bacterial bioreporters for arsenite detection, relatively few studies reported their response to arsenate levels. In this study, green fluorescent protein-based whole-cell Escherichia coli bioreporter was constructed for the measurement of both bioavailable arsenite and arsenate in water. The developed bacterial bioreporter has much higher sensitivity toward arsenate in inorganic phosphate-limiting medium compared to minimal medium. Both arsenite and arsenate were detected at 10 mu g/l within 2-h induction time period. Furthermore, the bioreporter was able to detect and estimate arsenic levels in groundwater sample. The results demonstrate that the constructed bacterial arsenic bioreporter is applicable in determining the concentrations of the most abundant two ionic forms of arsenic as opposed to analytical methods which gives total content of arsenic. Thus, a comparison of bioavailability of arsenic and its form with its total content would give an insight about the risk of toxicity and also planning of removal processes.