Dielectrophoretic measurements have been made on intact Micrococcus lysodeikticus and its protoplasts as a function of the ionic strength of the suspending electrolyte over the frequency range 20 Hz to 4 MHz. It is concluded that for frequencies above 1 kHz the standard theories accurately predict the observed dielectrophoretic behaviour. However, at low frequencies the theory is inadequate since it does not take into account the inhomogeneous and charged structure of the micro-organisms. It is also concluded that the low-frequency dielectrophoretic response of Micrococcus lysodeikticus is dominated by the electrical properties of the cell wall.