Abstract: Background and Objective: The aftermath of heavy usage of antibiotics could be harmful on the fish and aquatic environment by creating drug resistant bacteria and transferable resistance genes in fish pathogens. This study was conducted to evaluate the efficacies of whole lemon grass plant extracts on pathogens isolated from African mud catfish and examined the histological and haematological indices of sub-adult African mud catfish collected from private fish farms in Odogbolu, Ogun state, Nigeria. Microbial examinations, biochemical tests, DNA extraction and molecular characterization of bacteria were conducted. Materials and Methods: About 150 experimental farm catfish were infected with bacteria (Aeromonas veronii) with visible lesions on flesh and was subsequent treatment with lemon grass (Cymbopogon citratus) leaf extracts at concentrations of 2.5, 5.0, 7.5 and 10 mg g–1 and aquaceryl antibiotic 10 mg g–1. Cymbopogon citratus was extracted using ethanol, while qualitative and quantitative phytochemical screening were carried out after being concentrated. Fish samples were analyzed for haematological and histological profiles. Complete randomised design experiment was used for this study. Dissolved oxygen, pH and temperature were monitored weekly on the experimental farm. Results: Data were analyzed using analysis of variance. Phytochemicals revealed the presence of alkaloids, tannin, saponin, flavonoids and phenol with quantitative values of alkaloid at 6.78±1.00 mg g–1, flavonoids 3.22±1.00 mg g–1, saponin, tannin and phenol had 0.42±1.00, 1.68±1.00, 4.29±1.00 mg g–1, respectively. The haematological profile of fish post inoculation showed that White Blood Cells (WBC) increased significantly (p<0.05) compared with control. After treatment with plant extracts, the blood parameters including WBC, Haemoglobin (HGB), Red Blood Cells (RBC), Haematocrit (HCT), Mean Corpuscular Volume (MCV) and Mean plateletcrit Volume (MPV) increased significantly (p<0.05) at 2.5 mg g–1. The liver and the gills were normal after treatment with extract. Conclusion: This study showed that whole lemon grass plant improved the haematological and histological profile of inoculated fish.