This study presents the development of a QCM immunosensor for the detection of Listeria monocytogenes. A self-assembled monolayer (SAM) of thiosalicylic acid is incorporated for the covalent attachment of antibodies to the gold surface of the piezoelectric crystal. A non-Sauerbrey increase in frequency is observed upon exposure of such a crystal to specific antigen cells. This unexpected response is consistent with the obtained results and is shown to be specific. The sensor can detect L. monocytogenes cells in real time in solution to 1 x 10(7) cells/ml. The sensor is reusable more than 10 times without detectable loss in activity and shows negligible response to a non-specific pathogen, Bacillus cereus. The lifetime of the thiolated crystal was also investigated.