A single particle photometric detector is described for use with steric field-flow fractionation. By counting individual particles, the detector circumvents the difficult problem of calibrating detector response for particulate systems. The system is based on the detection of light from a He-Ne laser scattered by particles flowing through a thin flow cell. It was designed for monitoring particles in the 1-70 μm size range. The flow cell, which is easily constructed, has a total volume of 4.6 μl and an optical viewing volume of only 2.8 x 10-3μl. A low power compound microscope is a major component of the system. The performance characteristics of the detector are determined by using polystyrene latex beads of different diameters carried through the cell at different flow rates. Measurements are reported for relative signal response as a function of particle diameter, cell time constant as it depends on flow rate, and particle count in relationship to particle concentration. Examples of the detector's response are shown.