The San Patricio Municipal Water District is a wholesale supplier of water to cities and industry in south Texas, USA, with nine municipal and six major industrial customers. It operates three water treatment plants. The newest, Plant "C", is an MF plant rated at 7.8 mgd using Pall/Asahi PVDF membrane modules. The plant has been in continuous operation since January 2000. In June 2004 one rack (50 modules) was equipped with the Hach Company Medusa system for determining the turbidity of filtrate exiting each module. Turbidities are determined using a laser turbidimeter and multiplexing via fiber optic cables to detectors at each module. At the same time a pilot plant, one fullscale MF module is being used to define the sensitivity of the Medusa system in terms of pathogenic log removal. Membrane integrity in the pilot plant module has been intentionally destroyed and then challenged with 2.9 micron polystyrene beads as a surrogate for Cryptosporidium oocysts. The regulatory agency, the Texas Commission on Environmental Quality, observed the procedures. It is hoped to gain regulatory recognition of the Medusa system as a technique and as an operationally viable method for direct product turbidity measurement at the module level. The paper presents the details of the projects and experience gained from the pilot plant and full-scale plant operation. Tentative conclusions are reported.