Abstract: Sub-micron particles in fluid such as ultrapure water are detected or monitored by a simple apparatus in which a light beam from a coherent light source (1) is converged (2) in such a manner that the light beam is focussed in a stream (3) of particle-containing fluid, the light passed through the stream and diffracted by the particles is received by a photo-detector (4) which is positioned at an opposite side of the coherent light source with respect to the stream and substantially on an optical axis of the light beam, so that the number of particles in the stream is counted from electrical signals emitted by the photo-detector.