Investigation into the effects of Moringa oleifera leaf extract on bacteria (Aeromonas hydrophila) infected adult African mud cat fish (Clarias gariepinus) was conducted using twenty infected adults Clarias gariepinus and treated with M. oleifera leaf extract at concentrations of 25%, 50%, 75% and 100% respectively. Blood samples were collected (pre-inoculation, post-inoculation and after treatment) from fish in replicate and analyzed for haematological parameters vis-à-vis RBC (Red blood cell), WBC (White blood cell), PCV (Packed cell volume), MCH (mean corpuscular haemoglobin), MCHC (mean corpuscular haemoglobin concentration), MCV (mean corpuscular volume), Hb (haemoglobin), L (lymphocyte) and N (Neutrophil). It was observed that upon inoculation with A. hydrophila, all the haematological indices deviated to an abnormal range due to the inhibitory effects of the bacteria on the fishes. Results indicated no significant differences (P<0.05) among the parameters except white blood cell which shows significant difference at 50% treatment with Moringa oleifera extract. Infected fish responded to treatment better at 50% concentration Moringa oleifera leaf extract.