Abstract: The stream water quality of the Yalfan catchment of Iran has been studied intensively because of high organic pollutants loading to the Ekbatan Dam Lake. The influences of village`s wastewaters on the streamwater quality within the catchment were examined. The surface water has been contaminated with the faecal materials of man or animals. These pollutants could be contributed independently or in combination, to the very high levels in bacterial counts that were observed at this sampling site. All data include total and faecal coliforms counts, turbidity, pH, DO and COD at 11 different sites of the Yalfan River and demonstrate seasonal patterns for the period April 2003 to September 2004, which were divided on the dry seasons rainy seasons. The results indict that quality of the Yalfan River waters is generally better in the rainy seasons. But, the quantity of the wastes from human sources during the dry seasons is not very different to the rainy seasons. The total period analysis sheds no light on any significant trends in water quality over time within the year. The catchment situation`s resulting an important increasing of the COD, total coliforms and faecal coliforms at the end of Yalfan River direction that are in average 5, 10 and 13 times of upstream of the river, respectively. Conversely, dissolved oxygen level decreased along the river toward the end of direction. The averages of the total and faecal coliforms counts and so the COD throughout the sites were generally high of 1396 and 1185 MPN 100 mL-1 and 1209 mg L-1, respectively. This study generated some essential information of the Yalfan River pollutant sites that cause the microbiological contamination of the river as an important resource of Hamadan drinking water.