A quartz-crystal microbalance (QCM)-probe biosensor, based on thiol-derivatized E. coli lacZ gene probe was developed to detect low levels of specific microbial population in environmental samples. Firstly total bacterial DNA was purified from the lysate, and then polymerase chain reaction (PCR) amplification of a segment of the coding region of E. coli lacZ was done. The combination of the PCR amplification and non-radiolabeled gene probe on QCM could detect 10 fg of genomic E. coli DNA and as few as several viable E. coli cells in 100 ml of water, but noncoliform bacteria. This technology provides a sensitive method of detecting bacteria carrying lacZ gene coding for β-galactosidase products.