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Fresh fish in Nigeria is gaining acceptance as a result of knowledge of its nutritive value; however, it is mostly assessed by the consumers by mere looking. Fish that looks apparently healthy may not be actually healthy. Haematology, plasma biochemistry and serum are some good parameters for the assessment of fish health. This study evaluated the haematology, plasma biochemistry and blood enzymes of farm-raised African catfish, Clarias gariepinus, naturally infected with listeriosis in Oyo State. Fish were collected from 36 farms using stratified method. A total of 108 fish were collected where 37 were positive for listeriosis. The blood of the affected fish was used for this study. Haematology, plasma biochemistry and blood enzyme analysis were determined. Fish from Oyo zone had the highest PCV, Hb, RBC and MCHC with 29.33 ± 2.96%, 10.04 ± 1.04 dL, 3.55 ± 0.09 × 106/μL and 34.24 ± 0.74%, respectively; the highest heterocytes (39.00 ± 5.33%), eosinophils (4.00 ± 1.10%), basophils (0.50 ± 0.55%) and hetero:lymphocyte ratio (0.71 ± 0.14) were recorded in fish from Ogbomosho zone; WBC, platelets, lymphocytes and monocytes were highest in fish from Saki zone with 18.72 ± 0.44 × 106/μL, 224.00 ± 3.61 × 103/μL, 71.33 ± 1.53% and 3.33 ± 0.58%, respectively, while fish from Ibadan/Ibarapa zone had the highest MCV (163.40 ± 44.23 fL) and MCH (52.77 ± 14.95 pg). Total protein and globulin were highest in fish from Oyo zone with 6.61 ± 0.37 and 1.70 ± 0.38 g/dL, respectively, while albumin and A:G ratio were highest in fish from Ogbomosho zone. Fish from Ogbomosho zone had the highest AST, ALP, creatinine and glucose with 194.17 ± 10.11 μL, 232.17 ± 63.64 μL, 0.72 ± 0.19 μL and 277.50 ± 90.36 mg/dL, respectively; urea and cholesterol were highest in fish from Saki zone (8.30 ± 0.46 μL and 263.67 ± 3.51 mg/dL) while Ibadan/Ibarapa zone had the highest ALT (37.11 ± 14.72 μL). The study concluded that fish that are physically healthy which is the common factor used to judge the health status of the fish by majority of consumers do not mean that the fish are healthy. Haematology, biochemistry and blood enzymes of the fish infected with listeriosis are lower than the recommended levels. Therefore, further research should be carried out to detect a quicker way of examining healthy fish. [ABSTRACT FROM AUTHOR]