The freshwater crayfish of the Aras Reservoir is an important economic fisheries resource of West Azarbaijan, Iran. This study was conducted to evaluate the prevalence of parasitic infestation of Crayfish seasonally in this area in 2010. Among 390 different sizes of Astacus leptodactylus which were examined,  a range of ectocommensals or ectosymbionts from a number of different phyla including 9 orders and 11 classes  infested the different anatomic units of the surface and appendages such as  gills,  head, thorax, abdomen, walking legs, uropod, telson, antennae and antennulae of freshwater crayfish. Common groups such as peritrich ciliates, suctorian ciliates, free living nematodes, branchiobdellids, and algae, copepods, rotifers and oligochaetes have also been observed in association with freshwater crayfish.