Duplicate static 96-hour bioassays were conducted to determine the median lethal concentration for Oreochromis mossambicus (LC sub(50)) of 2 organostannous molluscicides, commonly used in fishponds: Aquatin and Brestan. O. mossambicus was more sensitive to Brestan. The acute toxicity of both Aquatin and Brestan ceased towards the end of 96 hours. The 24, 48, 72 and 96 hour LC sub(50) s were 4.01, 3.97, 2.95 and 2.58 ppm formulated product for Aquatin and 0.35, 0.18, 0.10 and 0.09 ppm for Brestan, respectively. The computed safe concentrations for Aquatin and Brestan are 0.30 and 0.01 ppm, respectively. Explore to lethal concentrations of Aquatin resulted in an immediate reduction in hemoglobin and hematocrit levels. Hemoglobin content was likewise lower in Brestan-exposed fish, whereas their hemotocrit level was higher than that of the control fish. Histological analyses of gills, intestine, liver and kidney showed pathological changes even in sublethal laevels tested. Damage became severe with increasing concentration of the pesticide. The behavir and symptoms exhibited by the fish and the physiology of hematological and histopathological changes are discussed.