The tsetse, Glossina morsitans morsitans, has two pairs of Malpighian tubules. Ultrastructurally, the tubules consist of three recognizable functional zones: the distal, intermediate and proximal regions, which are characterized by three distinct cell types: I, II and III respectively. The three cell types had numerous microvilli and basal infoldings: the microvilli of type I and II cells were associated with long mitochondria, while those of type III were devoid of mitochondria. Autoradiographic observations using (3H) glucose indicated that there was synthetic activity in the tubules, and that there was greater incorporation of the radiochemical in the distal and intermediate tubule regions than in the proximal region. The distal and intermediate regions have some functional similarities. With regard to primary urine formation in the tubules, it is suggested that the cell types I and II (and hence the distal and intermediate regions) are secretory, and the cell type III (that is, the proximal region) is absorptive. “Canaliculi” in the proximal region increase the surface area for absorption.