Black flies are blood-feeding insects that are distributed worldwide. Few studies, however, have reported the distribution and extent of medical and veterinary problems caused by these flies in Iran. This study aimed to identify the species of simuliids, based primarily on morphological characters, determine their spatial distributions, and identify their breeding sites in six provinces (Tehran, Mazandaran, Isfahan, Lorestan, Hormozgan and Khorasan-e-Shomali) of Iran from spring, 2010 to summer, 2012. A total of 914 larvae and 13 pupae were collected from sixteen areas in the above-mentioned provinces. Twelve taxa were found: Simulium alajense Rubtsov 1940, S. au reum Fries 1824 group, S. bezzii (Corti 1914), S. brevitarse (Rubtsov 1976), S. crassicaulum (Rub sov 1955), S. dahestanicum (Rubtsov 1962), S. ornatum Meigen 1804 group, S. paraequinum Puri 1933/pseudequinum Seguy 1921, S. paucicuspis Rubtsov 1947, S. transcaspicum Enderlein 1921, S. turgaicum Rubtsov 1940, and S. variegatum (Meigen 1818). Khorasan-e-Shomali province had the highest prevalence of Simulium species. A new record for Iran (Simulium dahestanicum) was discovered during the sampling.