A recombinant whole-cell bacterial sensor for highly selective and sensitive detection of the bioavailable methylmercury in the environment was constructed. The biosensor carries luciferase gene, luxAB, from Vibrio harveyi as a reporter under the control of the mercury inducible regulatory part of mer-operon from Pseudomonas K-62 plasmid pMR26. In addition, a merB gene encoding organomercurial lyase which cleaves the C-Hg bond of methylmercury to give Hg(2+ )was coexpressed in the sensor. The resulting bacterial sensor responded specifically to methylmercury, and the lowest detectable concentration of methylmercury was 10 pM with 1 ml sample in the optimized assay conditions. This detection limit is enough to detect this compound in many contaminated and some pristine environmental samples.