A quartz crystal microbalance (QCM) immunosensor was described for the detection of Salmonella Typhimurium with simultaneous measurements of the resonant frequency and motional resistance. The immunosensor was fabricated using protein A for the antibody immobilization. High-frequency impedance analysis indicated that the changes in resonant frequency and motional resistance (DeltaF and DeltaR) of the QCM were significant while the changes in static capacitance, motional capacitance, and motional inductance were insignificant. In the direct detection of S. Typhimurium in chicken meat sample, DeltaF and DeltaR were proportional to the cell concentration in the range of 10(5) - 10(8) and 10(6) - 10(8) cells/ml, respectively. Using anti-Salmonella-magnetic beads as a separator/concentrator for sample pretreatment as well as a marker for signal amplification, the detection limit was lowered to 10(2) cells/ml based on the DeltaR measurement; however, DeltaF was not related to the cell concentration. No interference was observed from E. coli K12 or the sample matrix.