It is demonstrated that contamination of water distribution systems with fungi is determined by the number and species composition of the mycoflora of waters supplying a given Water Treatment Plant (WTP) and the effectiveness of their removal in the unit processes of water treatment used. In the water distribution system examined, a significant number of microorganisms occurring in waters supplying the WTP was reduced in the water supplied to the system to 200 cfu l^-1 following sorption, coagulation, filtration and disinfection processes. Their number did not exceed 267 cfu l^-1 in the water phase in the distribution system while it was as many as 1000-5000-times greater in the biomass “suspended” in it. These organisms occurred sporadically in pipe sediments. Moulds, including species pathogenic and potentially pathogenic to humans and warm-blooded animals, constituted the mycoflora.