The gross and microscopic morphology of the gills of both Nile tilabia (Oreochromis niloticus) and sharptooth catfish (Clarias gariepinus) were described including light and scanning electron microscopy. The anatomical differences between the gill system of the two species were basically related to the geometry of the head and opercular cavities. Each species had four pairs of gills which were connected in a median interbranchial septum. In addition, a fifth rudimentary gill without gill filaments was found in catfish. The lengths and gaps between the gill arches in both species decreased medialwards. The gill rakers of tilabia were generally short and widely spaced compared to the long and narrow spaced rakers of the catfish. The gill filaments and lamellae of tilabia were longer than those of the catfish indicating a greater gill surface area in the former species. The catfish was additionally supplied by modified gills in the form of branched bulbous dendritic structures originating from the second and fourth gill arches. Scanning electron microscopy revealed that the surface of both gill filaments and lamellae was covered by polyhedral cells in tilabia and oval or elongated cells in catfish. These cells carried numerous microplicae which were better developed in tilabia. All gill parts, as revealed by light microscope, were nearly covered by mucous epithelium, however, the mucous cells were present on the lamellae of the catfish only. The rakers of both species demonstrated many taste buds and infiltrating lymphocytes in the epithelial covering. In catfish, in particular, several alarm substance cells were also scattered in this epithelium.