This study investigates the toxic potential of Iron wood, S. siamea to African Sharp tooth catfish, C. gariepinus (Burchell 1822) fingerlings. The bioassay used 360 fingerlings of Clarias gariepinus (mean total length, 19.7 ± 0.76 cm and mean weight 33.8 ± 1.62 g). Randomly selected 10 fish were exposed for 21 days to 0.0 mmg/L, 2.5 mg/L, 5.0 mg/L, 7.5 mg/L, 10.0 mg/L and 12.5 mg/L of leaf extract respectively each with a replicate. 96 h LC50 for leaves extracts was established at 31.62 mg/L using logit technique. Maximum Allowable Toxicant Concentration, MATC of the extract was established at 1.581 mg/L. Physicochemical parameters (pH and Dissolved Oxygen) of test media showed significant (P<0.05) concentration related reduction in values (Total Dissolved Solids and Conductivity) increased significantly (p<0.05) with increase in extract concentration. Temperature had nonsignificant (p<0.05) reduction at all extract concentrations. Blood analysis revealed significant (P<0.05) reduction in the blood parameters (Heamoglobin, Packed Cell Volume, Red Blood Cells, White Blood Cells, Platelet, Mean Corpuscular Volume and Mean Corpuscular Heamoglobin) of exposed fish. From Pearson correlation matrix, changes in physiochemical parameters of test media produced good statistical correlation with changes in blood parameters except for temperature. Thus changes in physicochemical parameters enhanced the toxicity of the studied plant to the test organism. Histology of tissues (gills, intestine and liver) of exposed fish revealed broad areas of inflammation, cellular proliferation, degeneration, cerebrobrachial damage etc. in exposed fish. The observed haematological and histological changes resulted in disease conditions (anaemia, leukemia, oedema, hypoxaemia etc.) in exposed fish. S. siamea was found to be toxic to C. gariepinus juveniles and is recommended as piscicidal plant for use in aquaculture.