A rapid and automated method for detecting bacteria is described. This method stands on simultaneous measurement of the changes in electrical impedance and turbidity of culture medium. The system is composed of the culture cell, detecting electrical circuit, A/D converter and microcomputer. The impedance change is measured by means of four-electrode technique and the turbidity change-is monitored using a pair of light emission diodes (LED) and CdS photo-conductive elements. The differential curves (the rate curves), which show the characteristic patterns specifying each type of bacteria and good reproducibility, are computed from the changes of impedance and turbidity. The standard differential patterns (the standard rate patterns) are calculated for each type of bacteria by averaging the rate curves several times. Identification is carried out by comparing the rate curves of unknown bacteria with the standard rate patterns. 102 samples of ten bacteria species (coliform bacteria) have been tested. 101 samples (99 percent) have been identified within 2-5 hours. 100 percent of test samples have been correctly identified within 5-8 hours. This experimental apparatus is rather simple compared with other automatic systems, such as the light scattering system or the radiometric method, and this method can be made available in biological routine test.