Turbidimeters and optical particle counters (OPCs) are used to monitor particulate matter in water. The response from these instruments is governed by the optical properties of the suspension and the instrument design. The recommended design criteria for turbidimeters allows for large tolerances that lead to variations in measurements from different instruments. OPCs provide size-specific information but may inaccurately size microorganisms or particles having optical properties different from those of the calibration particles. The authors evaluate the effects of optical design parameters in turbidimeters and OPCs. Performance data from two OPCs are presented. A multiple-angle light-scattering method was developed to obtain "optical signatures" from suspensions of monodisperse spheres and several microorganism isolates. Such "signatures" provide more optical information on waterborne particles and may also be used to identify specific types of microorganisms in real time. Includes 46 references, tables, figures.