We describe an instrument for single particle optical sizing, based upon small angle light scattering by individual particles. A colloidal dispersion is hydrodynamically focused into a narrow stream of 5 m width, which is illuminated over a length of 30 m by a perpendicular laser beam with a constant intensity across this detection volume, using a specially designed elliptical optical focus. As the particles pass one-by-one through the detection volume, the light pulses are detected under a scattering angle of 5°. The signals are stored in a multichannel analyzer and electronically processed. This set-up enables one to measure accurately the particle size and particle size distribution of both stable and aggregating colloidal dispersions. In this paper, we give some illustrative results for stable systems.