The ultrastructure of the Malpighian tubules of the final instar nymph of the mayflyBaetis rhodaniis described. Numerous Malpighian tubules enter the alimentary canal at the transition from midgut to hindgut. Each of them is made up of a cylindrical distal portion and a proximal narrow duct emptying into the gut. At the junction between the two regions, the cylindrical portion enlarges and curves, to continue into the thin duct. The wall of the cylindrical portion consists of primary cells characterized by typical features of the secretory/absorbing epithelia. These cells show a large polytene nucleus, an apical microvillated border, a high number of mitochondria and basal infoldings. The narrow duct is formed of large cells joined by highly convoluted junctions, and their cytoplasm is filled with electron‐translucent vesicles. The apical border of the cells of narrow duct lacks microvilli but bears several finger‐like extensions. Each narrow duct empties into a canal interposed between groups of gut epithelial cells, which constitute the transition area between midgut and hindgut. The Malpighian tubules ofB. rhodanirepresent an ancestral condition and their structural arrangement provides important information for tracing mayfly evolutionary trends.