Internal pipe surfaces within drinking water distribution systems (DS) are colonised by bacteria forming a biofilm. Detachment of cells from the biofilm leads to a deterioration of the microbiological water quality. This justifies the requirement for biofilm investigation in DS. This paper presents data on biofilm gained over a long period in the different areas of the Parisian suburbs DS. Two different systems were tested to incubate cast-iron coupons in DS to measure biofilm after colonisation in order to study biofilm. It shows that a simple and inexpensive system can be efficiently used for this purpose. Fixed bacterial biomass was routinely estimated, in this study, on cast-iron coupons by potential exoproteolytic activity (PEPA) and sometimes by epifluorescence microscopic enumeration performed after bacteria detachment from the support by sonication. Bacterial biomass in the biofilm ranged from 0.009 to 0.56 µgC cm−2 in the studied distribution systems. Comparing bacterial numbers in the water phase, on one hand, and fixed to pipe walls, on the other hand, showed that fixed cells were much more abundant than the suspended cells in DS (25 times in a 100 mm diameter pipe). The large set of data gained on biofilm within the Parisian suburbs DS allows us to demonstrate, on full scale data, the major role of disinfectant (chlorine) residual and biodegradable dissolved organic carbon (BDOC) as controlling factors of the biofilm in DS.