Countries suffering from water shortage problems use the intermittent mode of supply where water is pumped for few hours once or twice a week, which forces householder to store water in tanks located on rooftops or in reservoirs beneath the house. Water stored for several days may rapidly deteriorate imposing health risks on consumers. The first part of this study aimed at the examination of the microbial populations present in different household water storage tanks, and to examine the extent and the phenomenon of this microbial regrowth and determine the factors involved in it. The heterotrophic plate count (HPq of the household water storage tanks in Byblos city, Lebanon ranged from 5 CFU/ mL to 4xl04 CFU/ mL. A significant difference was not detected in the total bacterial count in the samples collected from the different types of tanks (black polyethylene, fiberglass, asbestos (etemite) and galvanized cast iron). Pseudormrns (P. aeru[!jna;a, 26.2%) was the dominant type observed, while Bacillus (B. amJdiqu{aciens, 9.63%) was the next dominant organism isolated from the storage tanks. Opportunistic pathogens such as P. aempjnaa (2 of 7 samples), E. sakazakii (2 of 7), E. a[§!}onrrans (1 of 7) and A eronvnas sdJri£t (1 of 7), in addition to the dominant bacterium Burkhdderia cepacia (5 of 7) were detected in the drinking water samples collected from Byblos city. The presence of such opportunistic pathogens could be attributed to inefficient chlorination. Thus the city drinking water is not of acceptable microbiological condition after reaching consumers due to the considerable occurrence of such microorganisms. However, P. aerugjnaa and coliforms were not detected in the UV treated drinking water samples. The effect of water retention time on bacterial regrowth pattern in the chlorinated city fiberglass tank was studied. Results obtained indicated bacterial regrowth with the dominant species being Pseudormnas spp. Bacterial exponential growth occurred during the