A laser diffractometer is described which employs a novel scanned array of optical light guides to record the angular intensity variation of light scattered radially by a particle throughout an arc of 180° in approximately 2.8 msec. This rapid response allows the study of individual particles moving in a liquid laminar-flow system at rates of up to ~5 × 10³ particles per min. Studies involving water-borne polystyrene latex microspheres of mean diameter 5.7 μm have indicated that by comparison with theoretical scattering characteristics derived from the Lorenz-Mie theory, individual sphere diameters may be estimated to within 2%. Preliminary studies of aqueous-borne spores of the imperfect fungi Penicillium chrysogenum and Aspergillus niger have demonstrated the potential of the technique in the analysis of biological microorganisms where the retention of specimen viability, lost in systems requiring specimen staining, is of considerable importance. By comparison with the Mie scattering characteristics for homogeneous spheres, estimates of spore size distributions and mean refractive indices have been determined, the latter values being 1.46 (±0.02) and 1.50 (±0.02) for the P. chrysogenum and A. niger spores, respectively.