exually mature specimens of the mosquitofish, Gambusia affinis (Baird and Giard) were exposed to a low concentration (0.2 mg/1) of the molluscicide Bayluscide for 15 days. The testes of these fishes, examined by the EM, have revealed severe cytological changes. The intertubular spaces were infiltrated binucleated cells,and different types of lymphocytes with degenerated mitochondria. The epithelial cells of the sperm follicles showed degenerated mitochondria and dilated endoplasmic reticulum. The round nuclei of the sertoli cells became flat and irregular in shape with less dense euchromatic material and degenerated nucleoli. The endoplasmic reticulum and the ribosomes became very few, and the
microfilaments became hardly defined. The sperm heads lost their normal arrangement and polarity in the sertoli cells. They became more round with irregular outlines. The acrosome and the cell membrane were degenerated. The mitochondrial sheath, in the midpiece, became separated from the axial filament and showed signs
of disintegration and disorganization