Biofilms on pipe walls in water distribution systems are of interest since they can lead to chlorine demand, coliform growth, pipe corrosion and water taste and odor problems. This study is part of an AWWA Research Foundation and Tampa Bay Water tailored collaboration project to determine the effect of blending different source waters on distribution systems water quality. This project is based on 18 independent pilot distribution systems (PDS), each being fed by a different water blend (7 finished waters blended in different proportions). The source waters being compared include groundwater, surface water and brackish water. These are treated in a variety of pilot distribution systems including reverse osmosis (RO) (desalination), both membrane and chemical softening and ozonation - biological activated carbon (BAC for a total of 7 different finished waters. The observations from this study have consistently demonstrated that unlined ductile iron was more heavily colonized by biomass than galvanized steel, lined ductile iron and PVC (in that order) and that fixed biomass accumulation was more influenced by the nature of the supporting material than by the water quality (including secondary residual levels). However, bulk liquid water cultivable bacterial counts (i.e. heterotrophic plate counts or HPCs) did not increase with greater biofilm accumulation, but results to date suggest high HPCs correspond with low disinfectant residual more than high biofilm inventory. Temperature affected biofilms also, and assimilable organic carbon (AOC) was important when residual was between 0.6 and 2.0 mg Cl2/l. An additional aspect of the work is that the potential of exoproteolytic activity (PEPA) technique was used along with a traditional so-called destructive technique in which the biofilm was scraped off from the coupons' surface, resuspended, and cultivated on R2A agar. Both techniques gave similar trends and relative comparisons among PDSs but culturable biofilm values were several orders of magnitude lower than PEPA values. Includes 16 references, figures.