The stable fly, Stomoxys calcitrans L., has two pairs of Malpighian tubules. Ultrastructurally, the tubules consist of three morphologically distinct zones: the distal, intermediate and proximal regions, which themselves are characterized by distinct cell types. The tubule cells have numerous mitochondria, microvilli and basal infoldings, which are associated with mitochondria. It is suggested that the distal and intermediate regions are secretory, while the proximal region is reabsorptive; and that the haemotophagous habit in S. calcitrans is probably correlated with the ultrastructure of its Malpighian tubules.