Escherichia coli is widely used as an indicator of fecal contamination of drinking water. E. coli concentration is routinely determined with culture based methods, even though in oligotrophic environment (e.g. drinking water) they may enter viable but not cultivable (VBNC) state. The aim of this study was to estimate the occurrence of VBNC E. coli in a water supply system in which culturable E. coli is not usually found. The sampling of water and biofilm (on pipes surfaces) from a water supply system was carried out over the period of more than one year. The system was supplied with chlorinated drinking water which was produced from chemically coagulated surface water and groundwater (Riga, Latvia). The biofilm samples were collected both with plug flow and completely mixed reactors during 2 – 3 week expose to drinking water. The water samples prior analyses were concentrated with tangential ultrafiltration method. All samples were analysed by cultivation on nutrient-rich selective culture media and by direct viable count (cells which elongate in nonselective media after treatment with nalidixic acid) combined with fluorescent in situ hybridization (DVC-FISH method). From 12 biofilm samples collected from 6 sampling sites at different times no cultivable E. coli was detected, however, all of the samples were FISH positive for E. coli and more than 50% of the samples contained DVC positive E. coli. Results showed that initially (1-2 weeks) VBNC E. coli is harboring the surfaces of the pipes (up to 500 cells/cm2) and then they are slowly released to water phase. In all biofilm samples positive hyperbolic correlation between biomass concentration and number of FISH-DVC E. coli was observed. We conclude that VBNC is important mode of E. coli “living” in drinking water networks and their fate is dependent on interaction between surfaces and water phase. The study contributes to understanding of ecology of E. coli in drinking water and provision on public health safety.