The objective of this study was to identify and assess the possible presence of coliform bacteria in Hamadan drinking water during drought seasons that generally influenced the water resources of Hamadan city. For this purpose, 464 water samples were collected during two periods (P1 and P2) between July 2003 to March 2005 and total and faecal coliforms bacteria (Coli.), dissolved oxygen (DO), temperature (T), pH and residual chlorine concentration (Cl) were measured. The results showed that Cl concentrations varied between 0.27 and 0.42 mg/l. The contaminated samples during periods P1, P2 and the total periods (PT) were 29.5, 12.5 and 21%, respectively, indicating the high coliform levels in water. The results indicated 61.5% of the samples contained Cl concentration between 0 to 0.5 mg/l. An inverse relationship was found between Cl concentration and coliforms of the samples. Contaminated groundwater of domestic wells, collected biofilms in old pipelines of the network, leakage of sewage water and weak chlorine concentration were the major factors that resulted in contamination of coliform-free water supplied from the reservoirs of the city. Further studies are necessary to better understand the exact reasons for the contamination detected during this research.