Benthic invertebrate species and their change was studied in Aras River during a hydro- biological research on the middle and terminal parts of Aras River that spanned the years 1995-1996 and 2005-2006. We found 91 species of benthic invertebrates ofwhich 85 species were identified during 1995-1996 and 49 species during 2005-2006. The highest rate of biodiversity was seen in molluscs with 19 species and  chironomid larvae with 17 species. Forty-two species had wide distribution and the remaining occurred only in special habitats. The biomass of invertebrates reduced from the upper reach of the river to the middle and lower reaches because of the changes in river bed from soil to sand. It is concluded that the formation of different habitats in different sections of the Aras River has a crucial role in the change observed in biodiversity of the benthic invertebrates.