The present investigation aims to illustrate the histological and ultrastructural differences of the liver and pancreas of a carnivorous fish, Schilbe mystus, and a herbivorous fish, Labeo niloticus. The histological examination of the liver
of both studied species revealed that the liver is mainly composed of a continuous compact field of the hepatocytes. The hepatocyte-sinusoidal structure of the liver of Schilbe mystus consists of the solid form and tubular one, while
that of Labeo niloticus consists of the tubular form and cord-like one.The ultrastructural examination of the liver of both studied species revealed that it is composed mainly of hepatocytes, Kupffer (macrophages) and to (fat-storing) cells. The hepatocytes of Schilbe mystus are characterized by the presence of numerous glycogen rosettes, numerous lipid droplets and a rough endoplasmic reticulum, while those of Labeo niloticus are characterized by the presence of numerous glycogen rosettes, a few lipid droplets and a roughendoplasmic reticulum. The hepatocytes of both studied species possess  numerous microvilli, which extend into the space of Disse or into the bile canaliculi lumina.
In both studied species, there is a prominent perisinusoidal space between the hepatocytes and blood sinusoids, the space of Disse. This space usually contains the Ito cells, which are stellate in shape and are characterized by the
presence of lipid droplets. The bile canaliculi of Schilbe mystus are of the intercellular type, while
those of Labeo niloticus are of both the intercellular and intracellular types. The histological investigation of the pancreas of both studied species revealed that there are three types: the compact, disseminated and intrahepatic
types. This pancreas showed that it consists of a highly lobulated gland of the compound acinar type. The acinar (pancreatic) cells are pyramidal in shape; they possess a basophilic cytoplasm and eosinophilic zymogenous secretory granules. The ultrastructural investigation of the acinar cells of both studied species revealed two cell types: the dark and light acinar cells. These cells are characterized by the presence of numerous zymogenous secretory granules and a
well-developed rough endoplasmic reticulum.