The effective electrical conductivity values for a variety of Gram-negative and positive bacteria have been determined in the frequency range 10 kHz to 100 kHz. This information enabled experimental conditions to be selected where micro-organisms of different species can be separated using dielectrophoresis - the movement of particles induced by non-uniform AC electric fields. Mixtures of micro-organisms of different species were separated locally on a microscope slide using micro-electrodes of polynomial geometry, and their physical isolation into two separate suspensions was accomplished using a dielectrophoresis chamber that incorporated interdigitated, castellated micro-electrodes.