A 5-year-old boy was admitted to our hospital because of an abdominal tumor at the right costal margin. Noninvasive examination confirmed the diagnosis of hydrops of the gallbladder using C-T Scanning, Echogram and X ray. The boy underwent the operation. The gallbladder measured 10.5X3X3cm in greatest dimension. About 1cm from the fundus of the gallbladder was a shallow external groove. The lumen was divided into two compartments by a complete transverse septum at the site of the groove. The proximal portion contained bile and the distal portion contained trans-parent serous fluid. Cholecystectomy was done. Histologically, the transverse septum was consisted of the smooth muscle and the epithe-lium of the gallbladder.