Effects of sub-acute exposure of pesticide aronil (propanil) on haematology and serum biochemistry of Oreochromis niloticus juvenile was investigated in the present study. Sub-acute doses (0.21 mg/l, 0.43 mg/l and 0.83 mg/l) were derived from 1/20, 1/10 and 1/5 of the 96h LC₅₀  (4.17 mg/l) of aronil, in which fish specimens were exposed for eight weeks. Control group was exposed to dechlorinated tap water.
Haematology and serum biochemistry of fish were examined at week 2, 6 and 8 of aronil exposure. Significant and dose dependent decreases in red blood cell, haemoglobin and haematocrit were observed in aronil exposed fish compared to the control. White blood cell, mean cell haemoglobin and mean cell haemoglobin concentration of the control were significantly lower compared to the exposed fish groups. Neutrophil and lymphocyte percentages were elevated significantly in 0.83 mg/l exposed fish when compared to other exposed fish and the control at the eight week. Basophils, eosinophils and monocytes were not observed in aronil exposed fish. Significant and dose dependent increases in glucose, protein, aspartate aminotransferase, alanine aminotransferase and triglyceride were observed in aronil treated fish with reference to the control while insignificant changes in cholesterol level was  observed in the exposed fish compared to the control. Aronil pesticide induced stress and had toxic effect on O. niloticus. Use of pesticide aronil should be regulated especially in farm lands along the coast and riversides to avoid the influx of pesticides into the aquatic ecosystems.