The ultrastructure of the first segment of the four-segmented Malpighian tubules of Cenocorixa bifida Hungerford (Hemiptera: Corixidae) is described. The first segment of each of the four tubules empties proximally into the midgut—hindgut junction. Its squamous epithelium is composed of a single layer of binucleated cells containing two types of mitochondria. The cells rest on a prominent basal lamina and possess short, shallow, widely spaced basal plasma membrane infoldings closely associated with large mitochondria and a few short blunt microvilli devoid of mitochondria projecting into the lumen. In sharp contrast, the fourth segment cells that are secretory in function, display numerous, long basal membrane infoldings that penetrate deeply into the cytoplasm and numerous long microvilli at the luminal border. The cytoplasm of the first segment cells contains dense bodies, small mitochondria, microtubules, vesicles, and vacuoles and is rich in smooth endoplasmic reticulum, giving rise to clear vacuoles, but rough endoplasmic reticula and Golgi apparati are sparse. The apical plasma membrane is covered with glycocalyx and displays coated pits pinching off coated vesicles. The apical cytoplasm displays numerous vesicles containing fine granular material, smooth vesicles, dense tubules, large vacuoles, primary lysosomes, and multivesicular bodies. The lateral plasma membranes are linked by extensive junctional complexes each composed of an apical smooth septate junction, intercalated gap junctions, and a basal heterocellular scalariform junction. The contrast in ultrastructural features between of the first segment and fourth segment cells suggests that the first segment is involved in reabsorption than secretion of inorganic and organic solutes.