An improved instrument is described for the characterization of particles suspended in gases by means of light scattering. It consists of (1) a sensitive right-angle optical system with a photomultiplier detector, (2) a coaxial flow system which facilitates variation of the particle concentration range, (3) a photometer which continuously records the average light scattering signal, and (4) a pulse analyzer for size discrimination of individual particles. The particle analyzer was calibrated with uniform polystyrene spheres from a 0.33 to 1.17 μ in diameter, employing air as a scattering standard. The scattering intensity was found to be approximately proportional to the square of the diameter. Resolving power was determined as a function of particle size, and the instrument was critically evaluated. A brief review of particle detectors is given. A versatile pulse height selector system is described. Some variations of the instrument are proposed.