In this paper, we describe the design and function of a microchip which is used to detect, analyse and count single micron-sized particles at high speed. The device uses multi-frequency electrical impedance together with single particle fluorescence spectroscopy. Impedance is measured using microelectrodes fabricated within a microfluidic channel. Optical measurements are made by focussing a beam of light into a detection volume of the order of the size of the particle. Particles flowing through the device are dynamically focussed into the detection volume using dielectrophoresis. The operating principle of the device is demonstrated by detecting and analysing fluorescent latex particles at high speed. (c) 2005 Elsevier B.V. All rights reserved.