Due to bacteria growth in drinking water distribution network water quality may deteriorate during transport from treatment plant to the consumer. Such water is called biologically unstable. Bacteria growth mainly occurs in biofilm. This paper reviews current literature and present some original results from research about biological stability and biofilm formation of drinking water in Riga water supply system. Results showed that in general drinking water contain high amount of microbially labile organic carbon. Bologically stability and biofilm formation of drinking water in Riga is dependent on source and treatment methods used. There is strong indication that during chemical precipitation of surface water bacterial growth in drinking water becomes limited by phosphorus. More research about factors that affect bacteria growth and biofilm formation is needed.