A new turbidimeter utilizing the laser diode as the light source has beendeveloped by the Hach Company of Loveland, Colorado, for the monitoring offiltered water quality. Although not approved by the Environmental ProtectionAgency for reporting purposes, this instrument has a 2 order of magnitude greaterresponse to turbidity when compared to tungsten and LED turbidimeters and rivalsthe sensitivity of particle counters. The objectives of this study were: todetermine if the laser turbidimeter had significant resolution below 0.1 NTU inorder to observe changes in water quality for the purpose of filter optimization in apilot-scale and full-scale drinking water treatment plant; and, to verify thatthe laser turbidimeter was as sensitive as a conventional particle counter tochanges in water quality during the three stages of a filter run (ripening,steady state, and, breakthrough). Includes 10 references, figures.