This study was taken to examine the ultrastructural changes of the gill and kidney of female tilapia (Oreochromis niloticus) living in 0‰, 10‰, 20‰, and 30‰salt concentrations, respectively, by scanning and transmission electron microscope. The ultrastructure by scanning electron microscope (SEM) indicated that the gill secondary lamella of tilapia exposed to seawater, were characterized by rough convoluted surfaces during the adaptation. Transmission electron microscopy(TEM) indicated that mitochondria in chloride cells exposed to seawater, were both large and elongate and contained well-developed cristae. TEM also showed the increased chloride cells exposed to seawater. The presence of two mitochondria-rich cell types may regard to their pos-sible role in the hypoosmoregulatory changes during seawater-adaptation. The kidney tissues in freshwater-adapted tilapia contained most of small, circular mitochondria, and mitochondria had less electron-dense cy-toplasm. The ultrastructure by TEM indicated that the kidney tissues of tilapia exposed to seawater, were characterized by the increased mitochondria during the adaptation. TEM also indicated that mitochondria in kidney tissues exposed to seawater, were both spherical and elongate and contained well-developed cristae. The kidney tissues in seawater-adapted fish contained most of large, elliptic(elongate) mitochondria and mitochondria had more electron-dense cytoplasm.