Freshwater crayfish (Astacus leptodactylus) is an important economic fisheries resource of Aras reservoir, Iran. The infection of freshwater crayfish by gram negative and gram positive bacteria are common in natural and cultural environments. These bacteria are considered as secondary disease agents or opportunities. This study was conducted to investigate infestation condition on 70 freshwater crayfish regarding the bacterial agents by culturing from haemolymph in aseptic condition on Blood Agar, Trypton Soya Agar (TSA) and Cytophaga Agar and was performed by biochemical tests. In this study Aeromonas hydrophila, Staphylococcus aureus, Micrococcus luteus and Flavobacterium johnsoniae were isolated and determined. According to the results which was performed for first time on Aras reservoir freshwater crayfish, the percentage of crayfish which carried the bacteria in haemolymph were variable (20-58/5%). Also, the gram negative bacteria particularly Aeromonas hydrophila were dominant among the bacterial isolates from crayfish.