In this research the affect of A. hydrophila on pathogenesis of Crayfish A. leptodactylus was carried out. This study was designed in three groups as treatments and one group as control in triplicate with 20 Crayfish in each glass aquarium. The treatments have exposed to A. hydrophila. The concentrations of the bacteria in treatments were      , and   CFUmL^-1 respectively. The untreated control group was disinfected by oxytetracyclin at concentration of 100 ppm for 24 hours. The haemolymph samples were withdrawn for measuring of THC and TPC within interval hours (2, 6, 12, 24, 48, 96, 144, 240 and 336). The Crayfish samples were then fixed in Davidson fixative. The results showed that the differences of THC value between treatment 4 (95.33±23.16) and control (1.13±.45) were significant (P<0.05) as well as control and treatments 3 (35.33±16.16) and 4 (95.33±23.16) during 2 initial hours. In case of THC the significant difference (P<0.05) was observed between control group (72.33±15.04) and treatment 3 (13.13±5.85) after 48 hours. During 240 hours, also the difference of THC value between control group (80.33±24.94) with treatment 2 (14.67±3.51) was significant (P<0.05). The finding of TPP value showed that there was no significant difference between control group and treatments in during mentioned times (P<0.05). The result of histopathology in heaptopancreas and gill sections showed that hemocyte aggregation and necrosis within pyknosis of nucleus. In lower concentrations of bacteria no pathological changes of heart were observed. In digestive tract no changes were appeared in treatments, but in concentration of 3×10⁸ CFU ml^-1 a low aggregation of hemocytes was revealed.