Three groups of O.niloticus fish, the first was injected with Aermonas hydrophila, the second injected with formalized whole cell bacterin and the last with phosphate buffer saline (PBS). Red and white cell counts, serum parameters denoting liver and kidney functions were measured over a 4-weeks period. As a results of infection, most of the clinical signs and lesions indicating Aermonas septicaemia were developed, during which red cell count, haemoglobin and haematocrite values were decreased, however, white cell counts showed an increased level during the first 9 days post-infection followed by a gradual decrease up to the end of the experimental period. Gradual insignificant increase in lymphocytes, monocytes and oesinophils was noticed in infected fish group during the first 9 days post-infection, followed by a significant progressive decrease up to the end of experimental period. Significant increase in serum bilirubin, blood urea nitrogen was recorded in infected fish group, however, a significant decrease in serum proteins, cholesterol and serum glucose was also found. On the other hand, the measured parameters in bacterin infected group were mostly close to those of control group except for the recorded increase in white cell count, especially lymphocytes and the insignificant increase in serum total proteins.