The thesis “Biological stability in chlorinated drinking water distribution networks” investigates the factors and processes in the drinking water distribution network (DN), which could promote bacterial growth. The thesis is particularly focused on bacterial growth in chlorinated DN, since in previous works biological stability was studied mostly in non-chlorinated DN. While generally disinfection is meant to reduce bacterial concentrations, there is a hypothesis that by killing a significant fraction of bacteria and oxidizing organic matter, chlorination could create an environment favorable to bacterial growth. Riga drinking water supply system was suitable for such investigation, based on the fact that chlorination is common practice in Riga. Moreover, large and complex Riga drinking water DN is supplied from chlorinated surface water, artificially recharged and natural groundwater, which gave a possibility to study an influence of chlorination and different water sources on bacterial growth in the network. The thesis consists of two main parts – testing and optimization of the methods for quantification of the viable biomass specifically for chlorinated samples, and two case studies, which investigated Riga DN to evaluate biological stability and to identify the reasons of bacterial growth. A general pipeline for SYBR Green I and propidium iodide (SGPI) staining method’s optimization for flow cytometric (FCM) measurements has been developed during the present work. Adenosine triphosphate (ATP) measurements were tested specifically for chlorinated samples, and used for biomass characterization of the distribution network samples. It was discovered that ATP was released from bacteria with high sodium hypochlorite dose, and extracellular ATP could be consumed by bacteria as a phosphorus source, thus contributing to biological instability of water. Application of these methods in the real chlorinated drinking water supply system showed temporal and spatial biological instability in the network. A modified microorganism growth potential approach allowed to determine growth-limitation during drinking water treatment and distribution, caused by various nutrients. It was demonstrated that carbon, released during chlorination of natural groundwater, could lead to increased bacterial growth in the DN. Moreover, we found that the main growth-limiting nutrients were different for the different types of water: treated surface water was phosphorus-limited, while carbon was the main growth-limiting nutrient in groundwater that contained phosphorus in excess. We overall conclude that nutrients released during chlorination is not the main reason of instability in the DN, but it creates favorable conditions for bacterial growth, whereas different water types with different growth-limiting compounds can pose a high risk of biological instability in the DN.