The study described herein was carried out on a technical scale on a water supply system servicing approximately 350 thousand inhabitants. The total length of the water mains and distribution pipelines in the area under review approximates 1 744 km and 37 264 water supply connections. The greater part of the water supply area is compact, and a region (a maximum located to the south) is clearly separated from the rest. Because the Waterworks Company, which manages the examined SDW, does not exploit its own intakes and water treatment plants, it has to purchase the water from two external suppliers (D-1 and D-2). The studied area is supplied with water from water mains through 21 points (P1-P21) which, in many cases, are also points of water pressure reduction (reduction stations) or elevation (pumping stations). In 8 of the 21 points of purchase, through appropriate installations, phosphate corrosion inhibitors are dispensed to the water. In an article, the microbiological evaluation of deposits collected from five points in the water distribution system was conducted. The analyzed deposits were located at three different materials: cast iron, plastic, and asbestos cement, which in terms of age ranged from 3 to 40 years. Samples of deposits were collected using the same method for various months during one calendar year. Bacteriological analysis of samples included determination of: total number of mesophilic bacteria (growing at 37°C), psychrophilic (growing at 22°C), coliform, faecal coliform (thermotolerant) or E. coli, fecal streptococci, Clostridium perfingens, bacteria of the Pseudomonas genus, bacteria precipitating iron, anaerobic bacteria sulfate reducing (SRB-initiate corrosion processes), actinomycetes and fungi which are the possible contamination of the soil. Results of microbiological testing showed that the surface of the pipes developed biofilm, which is a significant reservoir of microorganisms, which is a phenomenon often occurring in water distribution systems. The analysis showed a variety of deposits in terms of microbiological, depending on the type of material which they were originally placed on. In the case of sediment samples taken from the plastic pipe, the size of the bacteria grown in the two temperatures was several times higher than in the case of samples taken from the cast iron pipe. In the case of the water sample taken from the plastic, the size of bacteria grown at both temperatures exceeded the limit values for water several times.