An impedance splitting method was used to detect Bacillus stearothermophilus in suspension and attached to stainless steel surfaces. The effects of bacterial metabolism on the impedance of the culture medium and the ionic layers of the measuring electrodes were recorded using the BacTrac 4000 microorganism growth analyser. Impedance changes were measured at 55 degrees C. Seven of the eight media produced changes in the electrode impedance (E-value) and all media produced negligible changes in the impedance of the culture medium (M-value). Good correlations were obtained between cell numbers and the E-value measured over 18 h (r > 0.9) for the two strains of B. stearothermophilus tested in trypticase soy broth. The E-value correlations were used to estimate the numbers of both vegetative and spore forms of B. stearothermophilus as either planktonic or adhered cells. For the detection of B. stearothermophilus using impedance, only methods where the E-value impedance is recorded, can be used.