Problem description: Complicated processes occurring during drinking water distribution lead to water quality deterioration. One of the most important problems is microbial growth due to its impact on hygienic, aesthetic water quality and maintenance operation of distribution systems. The growth may occur in bulk water, suspended solids, loose deposits and pipe wall biofilm. During last few decades, studies have been conducted into each phase, especially bulk water and pipe wall biofilm. However, no research has taken all aspects into account. The objective of this dissertation is to evaluate the microbiological water quality in drinking water distribution systems integrally by considering quantitative and qualitive contributions of all phases. Research questions of this thesis were defined after reviewing literature in Chapter 2. Investigations in this thesis: As the most widely used approach by water companies, bulk water samples from locations in distribution systems were taken regularly to monitor distributed water quality. The study of biomass monitoring in drinking water distribution systems (DWDS) evaluated and compared methods of heterotrophic plate counts (HPC), Aeromonas, adenosine triphosphate (ATP) and TCC, with a focus on ATP and TCC (Chapter 3). Results obtained from two biological stable distribution systems over one year demonstrated that the TCC in the distributed drinking water averaged between 0,5-3,2x105 cells ml1. In both systems, the biomass differences measured by ATP were not significant. TCC differences were also not significant in treatment plant 1 (TPI). but decreased slightly in treatment plant 2 (TP2). TCC values were found to be higher at temperatures above 15 °C than at temperatures below 15C. Furthermore, a correlation study using several parameters describing the biostability of water found no relationship between TCC and HPC, TCC and Aeromonas. Also no relationship, as reported in other literature, was found between TCC and ATP. How