Abstract: A liquid flow cytometer for counting and classification of particles such as biological cells and other microscopic particles in liquids that involves an optical system for irradiation and detection in a metering zone (7), thus enabling optical dispersion and fluorescence caused by the particles to be simultaneously detected when the particles pass the metering zone, includes in a first beam path (S.sub.1) a mirror reflector (9) with its focal point in the metering zone (7), a quarter-wave plate (8) between the metering zone (7) and the mirror reflector (9), a lens (6) with its focal point in the metering zone (7) and located on the opposite side of it together with a dichroic mirror (5) behind the lens and provided in such a manner that it gives reflection to a second beam path (S.sub.2). In the second beam path (S.sub.2) a beam splitter (4) is provided between the dichroic mirror (9) and a known per se light source (1), and produces a third beam path (S.sub.3).