A differential light scattering photometer has been developed for rapid size analysis of single particles in flow. A fluid stream carrying individual particles in single file intersects a focused laser beam at the primary focal point of an annular strip of an ellipsoidal reflector situated in a scattering chamber. The light scattered from polar angles theta = 2.5-177.5 degrees at azimuthal angles phi = 0 and 180 degrees , spanning a circle of 355 degrees , is reflected onto a circular array of 60 photodiodes. The signal processing electronics and computer storage can accept 32 signals/particle at rates up to 1000 particles/sec. Photometer performance is tested by comparing measured responses from individual spherical particles with angular scattering patterns calculated for the particular detector geometry. These patterns exhibit the required symmetry in the two half scattering planes. Response measurements for eight samples with particle diameters of 1.1, 2.7, 5.0, 7.9, 10.0, 12.5, 15.6, and 19.5 microm are consistent with calculated size-response curves. The composition of a mixture of five components with particle diameters of 1.1, 5.0, 10.0, 15.6, and 19.5 Am is determined from an analysis of light scattering measurements at various forward-scattering angles.