Anatomic variations and congenital anomalies involving the gallbladder position, shape, and number are frequently encountered on routine abdominal imagings and at surgery. However, most have no clinical significance, but their recognition is important because they may predispose to gallbladder diseases, serve as a potential source of confusion and diagnostic pitfalls for radiologists and surgeons, and increase the risk of inadvertent injury during biliary tract surgery or intervention. We observed an intra-mesocolic gallbladder found unexpectedly during the cholecystectomy in a 65-year-old male patient who was being operated on for acute calculous cholecystitis. An abdominal ultrasonography and computed tomography scan reported no anomalous or malpositioned gallbladder pre-operatively. As the location of this organ could not be definitely clarified in his previous operation elsewhere, we performed an explorative lapa- rotomy. There was no gallbladder at the normal position. The organ was found embedded deeply within the proximal portion of the transverse mesocolon, and then it was successfully excised. We established the diagnosis of an ectopic gallbladder in mesocolic position.