In vivo acute and chronic toxicity effects of pendimethalin were carried out on the haematology and biochemical parameters of juvenile Clarias gariepinus subjected to different acute (0.00, 0.25, 0.50, 0.75, 1.00 mg/l) and chronic (0.00, 0.05, 0.10, 0.15, 0.20 mg/l) concentrations of pendimethalin for 4 and 28 days, respectively. As concentration of the toxicant increased in both assays, there were significant (P < 0.05) increase in red blood cells (RBCs), haemoglobin (HB) and mean corpuscular haemoglobin concentration (MCHC), while the white blood cells (WBCs), packed cell volume (PCV), mean cell volume (MCV) and mean cell haemoglobin (MCH) significantly decreased compared to the control. Alterations in biochemical parameters in C. gariepinus tissues during both acute and chronic exposures exhibited similar trend of either increase or decrease in activities as the concentration of toxicant increases. In both exposures, the activities of aspartate amino transferase (AST), alanine amino transferase (ALT) and malondialdehyde (MDA) significantly increased (P < 0.05) in the blood, gill and liver, whereas the activities of lactate dehydrogenase (LDH), superoxide dismutase (SOD), acetylcholinesterase (AChE), glucose and protein in these tissues were significantly inhibited (P < 0.05) compared to the control groups, though the magnitude of induction or inhibition in each tissue differs. The study shows that exposure of C. gariepinus to pendimethalin toxicity could cause oxidative stress and alterations in the haemato-biochemical profile of the fish. Therefore, the use of pendimethalin should be minimized and discouraged especially around aquatic bodies so as to prevent adverse effects on the fauna inhabiting them.