One of the most frequently obnoxious fishing practices in Nigeria water bodies is the indiscriminate use of toxic plant materials in catching fish, Lawsonia inermis leaves extract has been a potent fish poison used by artisanal fishermen. There is dearth of report on the toxic potential of I. inermis on the physiology of T. zillii. This study investigated the 96hr, acute toxicity of L. inermis leaves and its consequences on the haematological and biochemical parameters of Tilapia zillii fingerlings in a static bioassay. Definitive concentrations of 0.00 (control), 0.20, 0.30, 0.40, 0.50 and 0.60 g/L of I. inermis extract were used. Ten fingerlings (n=10) were stocked in each of the experimental tanks. The 96hr LC₅₀ of L. inermis on T. zillii was 0.38 g/L. With the exception of the significant alterations (p<0,05) in white blood cells count, there was no significant changes (p>0.05) in the levels of other haematological parameters (packed cell volume, red blood cells, haemoglobin concentration, mean corpuscular volume, mean corpuscular haemoglobin, mean corpuscular haemoglobin concentration and differential counts) compared with the control. There was a significant concentration dependent elevation in the levels of biochemical parameters; alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, total protein, albumin, high density lipoproteins, low density lipoproteins and lactate dehydrogenase relative to the control. The result of the present investigation revealed that crude leaves extract of I. inermis has detrimental effects on survival, haematology and biochemistry of the T. zillii fingerlings. The use of I. inermis to harvest fish in aquatic systems should, therefore, be discouraged.