Adult sexually mature male specimens of the mosquito fish, Gambusia ajfinis (Baird & Giard) were exposed to a low concentration (0.2 mg/1) of the molluscicide Bayluscide for 15 days. Semithin and ultra-thin sections of the liver, of both control and treated specimens, were examined by light and electron microscopes. The livers of the control fish showed that the typical hepatocyte has hexagonal shape and a large single euchromatic nucleus. Its cytoplasm contains rough endoplasmic reticulum, which is concentrated mainly around the nucleus and subjacent to the cell membrane. Many round, oval or rod-shaped mitochondria, a considerable amount of glycogen granules, in addition to rounded prominent fat globules are observed in the hepatocyte cytoplasm. The blood sinusoids are lined by endothelial cells, whereas numerous microvilli extend from hepatocytes into the sub-endothelial space (space of Disse). The bile canaliculi are formed at the junction of the hepatocytes.Liver cells of treated fish showed detectable changes from that of controls. The changes include reduction of rough endoplasmic reticulum (RER) and glycogen granules, as well as mitochondrial degeneration. The primary and secondary lysosomcs increased in number and became variable in size and shape. The hepatocytes also showed vacuoles of variable sizes and shapes and prominent leached fat globules. In addition to the previous changes, the severely affected cells showed evidence of nucleus and cell membrane degeneration with large cytoplasmic vacuoles communicating with each other.