Vernonia amygdalina has recently been linked to insecticidal and pesticidal properties that could replace the harmful agrochemical pesticide usage around the aquatic environment and to aquatic inhibitors such as fish. This study aimed to determine the effect of sub-lethal concentrations of Vernonia amygdalina (bitter leaf) on testes of Clarias gariepinus (African catfish) juveniles. The fishes were exposed to 0.00, 0.40, 0.80 and 1.60 g/L graded concentrations of aqueous crude leaves extract of Vernonia amygdalina for two months. The organ (testis) changes in antioxidant biomarkers’ histology, histomorphometry, and somatic indices compared with the control. There was the reduction in SOD, MDA and GSH activity as the concentration of the toxicant increased compared with the control. Mild interstitial oedema, mild tubular germ cell, interstitial cell depletions, severe depletion of seminiferous luminal content and sloughing off of the seminiferous tubular boundary tissue were observed in various concentrations of Vernonia amygdalina compared with the control. There was an increase in testis somatic indices as the plant extract concentration increased compared with the control. Bitter leaf extract was shown to have a toxic effect on Clarias gariepinus juveniles. As such, the pesticide use of the plants near the aquatic environment should be discouraged.