This study examined the toxicity effect of Kigelia africana on Nile tilapia (Oreochromis niloticus) juveniles. The corresponding effects of this plant extract on the health status of the O. niloticus were similarly studied using their haematological and histopathological profiles. The experiment was carried out at the Hatchery Unit of the Federal University of Agriculture, Abeokuta fish farm. Fish were acclimatized for one week and fed twice daily at the rate of 3% body weight. Water in the culture medium was replenished daily. A total of 150 juveniles of O. niloticus were exposed to concentrations of 0.00, 1.75, 2.50, 3.75 and 5.00 g L −1 aqueous extract of Kigelia africana set up in three replicates. This toxicity study showed that aqueous bark extract of K. africana caused significant behavioural changes in O. niloticus. Recorded values of the water quality parameters showed significant difference (p < 0.05) across the treatments. The haematological indices of the fish were also observed to be affected with increasing extract concentration, compared to the control treatment (0.0 g L −1). Similarly, histology of the liver and gills showed variations in distortions and damages to the tissues; with observed severity increasing with increase in extract concentrations. This study suggested that the 96-h LC 50 of K. africana could be greater than 5 g L −1. The study concluded that caution must be taken in the disposal of this plant in water bodies as extended exposure time and at higher concentrations could pose adverse effects on the stock of juvenile Orechromis niloticus.