Transmission electron microscope was used to reveal the reserve bladder system of the adult Cynodiplostomum azimi in experimentally infected rats. It was shown that this system consisted of a number of lacunae. The lacunar lining appeared as syncytial epithelium containing many nuclei, mitochondria, dense secretory bodies, Golgi complex and bundles of smooth muscles. The outer surface of the excretory epithelium was highly folded and lamellated. These lamellae were continuous with the excretory epithelium. Small lipid droplets were observed within the excretory epithelium, while larger ones were associated with the lamellae. The large lipid droplets were released in the lacunal lumen after the rupture of the lamellae surrounding them.