Bacterial activity and growth were monitored by following the changes of electrical impedance of cultures in liquid media. The signal is expressed automatically as a curve similar to growth curves produced by other methods. The technique offers a new, rapid and sensitive means of detecting active microorganisms and is potentially the basis of rapid automated systems in this field. The impedance changes indicate that the micro-organisms metabolise substrates of low conductivity into products of high conductivity and that the changes are due to the activity of the micro-organisms rather than increase in their numbers.