Abstract: An apparatus for hydrodynamically focusing particles to permit counting and classification of the particles, which apparatus includes a flow nozzle terminating in a capillary nozzle that forms a thin stream containing the particles and having a circular cross section with the particles oriented along one axis of the thin stream, and a jacket pipe enclosing the capillary nozzle for providing an entraining stream for the thin stream of particles leaving the capillary nozzle, is further provided with a further flow nozzle located downstream of the capillary nozzle and presenting a circular cross section for receiving the stream leaving the capillary nozzle, and changing its cross section from circular to rectangular such that the stream exits from the further nozzle with a rectangular cross section, a device directing a laser beam normally to one flat side of the rectangular cross section at a location downstream of the further nozzle, and a radiation detector disposed for detecting radiation resulting from impi