Scanning electron microscopic studies of the gills ofRhinomugil corsula andSicamugil cascasia reveal interspecific variations in distribution, density and architectural plan of the mucous glands and the cytochemical nature of the mucus secreted by them. The gill arch and gill filament epithelium ofRhinomugil corsula contains numerous mucous cells, butSicamugil cascasia has few mucous gland openings on its gill arch and gill filament epithelium. In both the mullets the secondary lamellar epithelium lacks mucous gland openings, which is discussed. The presence of blue, blue-red, red and red-blue mucous cells [AR-pH 2·5 periodic acid-Schiff] in the gills ofRhinomugil corsula signifies the acid and neutral nature of the glyco-protein of the mucus secreted by them. However, the presence of only blue mucous cells indicates the presence of acid glycoprotein in the mucus of the gills ofSicamugil cascasia.