Dielectric spectroscopy can be used as an on‐line method for the measurement of the formation of biofilms. The formation of a biofilm by the organismKlebsiella rubiacearumwas measured by registering changes in the capacitance in the frequency range 0–1–10 MHz using a 4‐terminal gold pin dielectric spectrometer. Removal and prevention of biofilms by electrolytic cleaning pulses was investigated. The effect of the addition of the biocides cetrimide, chlorine and glutaraldehyde was studied. The usefulness of the method in the on‐line control of biofilms was demonstrated in an experiment in which the biofilm level was controlled by the automated addition of chlorine in response to appropriate changes in the electrical capacitance.