This study was carried out to determine the toxicity of tamarind seed husk powder to Nile tilapia (Oreochromis niloticus). The range – finding test was conducted twice in order to get reliable values that could be used for the definitive test. The toxicant (tamarind seed husk powder) was introduced at varying concentrations of 0mg/l, 2mg/l, 4mg/l, 6mg/l , 8mg/l and 10mg/l in the first range- finding test, while it was introduced at concentrations of 1mg/l, 2mg/l,3mg/l, 4mg/l and 5mg/l in the second range- finding test. However, in the definitive test, the toxicant was introduced at varying concentrations of 1.2mg/l, 1.4mg/l, 1.6mg/l, 1.8mg/l and 2.0mg/l. From the definitive test, the Median Lethal Concentration of tamarind seed husk powder to Oreochromis niloticus was found to be 1.52mg/l. The study showed that Oreochromis niloticus juveniles exhibited initial erratic movement, rapid opercular movement, skin discoloration and loss of reflex. The water quality data were analyzed by subjecting them to one way Analysis of Variance (ANOVA).Thus, the study indicates that tamarind seed husk powder is toxic to Oreochromis niloticus juveniles.