The pollution of aquatic ecosystems by sodium pentachlorophenolate (Na-PCP) has assumed serious proportions due to their toxicity and accumulative effect in organisms and environment. The objective of this experimental study was to examine and quantify the histopatological effects of Na-PCP on the structure of hepatopancreas and gill, as two most important organs targeted by phenol entries, in the freshwater snail, Holandriana holandrii. Results indicate serious structural changes in tissue and epithelial cells of both organs and were accelerated with increase of Na-PCP concentrations and more pronounced related to exposed period. In hepatopancreas, tubular lumen, vacuole surface and number per tubule, exudate area in the tubular lumen, all increased with increase in concentration. Opposite trend of decreased height was observed for digestive and basophil cells. Significant gill structural changes appeared as a progressive necrosis at epithelial cells and the epithelial lining was interrupted, while connective tissue exhibited vacuolization, formation of large irregular spaces and damaged chitinous rods. Histomorphometrical approach in quantification of toxicant effects on the organism tissue could be used in legislative prevention of hazardous effects in freshwater ecosystems and H. holandrii may serve as a reliable and sentinel indicator of freshwater deterioration.