The urinary bladder of the Pacific cod,Gadus macrocephalus was investigated. A pair of ureters was found to unite and form a sac-like protrusion on the posteroventral aspect of the gonad. This protrusion was confirmed as the urinary bladder based on its histology and the presence of ammonia and urea in the fluid within this structure. The urinary bladder consisted of a central lumen and two lateral expansions found on the right and left sections. The luminal epithelial cells of the Pacific cod urinary bladder, unlike those of other animals, were characterized by microvilli on the free surface, and the presence of a number of vesicles and mitochondria across the apical portion on the cells. This suggests that the luminal epithelium of the urinary bladder might be actively engaged in transportation of water or other materials from the urine. The Pacific cod urinary bladder may therefore be closely associated with osmoregulation by reabsorbing water from the urine. Ammonia-N and urea-N levels of 65–213 μg/dl and 1.5–3.5 mg/dl were measured in the fluid which filled the urinary bladder.