Strobilocercus (Cysticercus) fasciolaris, the larval form of Taenia taeniaeformis, in the liver of a two-year- -old male pet rat has been described. A bladder containing the larva was filled with translucent, opalescent fluid. On opening the cyst, a coiled, 8-centimeter-long larva with an intussuscepted scolex was found. The elongated cestode had a juvenile segmentation and a vesiculiform caudal region. Histopathology of liver sections revealed deposits of bile pigments in plasma of the liver cells (icterus) and extensive degenerative changes in liver lobes adjacent to the cyst wall. Maturating in the rat, the bladder mechanically damaged the liver by pressing on the bile duct and thus causing icterus, whereas the secreted toxins damaged hepatic cells (steatosis hepatis). The diagnosis in naturally infected rats is usually difficult and not applied in practice.