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Algae are the most important group of organisms which considerably affect the water quality. Although algae constitute one of the fundamental parts of the aquatic food chains, only few researches deal with the freshwater algae of Iran. Filamentous algae are important components of the river vegetation. The Balikhlou River is utilized as a source to obtain drinking water for the city of Ardabil, therefore, the algae flora of this river was studied and a total number of 18 taxa of filamentous algae were determined, of which nine species were belonged to the Cyanophyceae class of Cyanobacteriae, while five speciesof which were belonged to the Chlorophyta. In addition, four species were recorded for the first time from Iran, i.e., Audouinella eugenea, Arthrospira gigantea, Homoeothrix janthina and Tychonema bourrellyi.