Two pseudoplesiopine species, Pseudoplesiops rosae and Pseudoplesiops howensis, were found to possess a highly specialized urinary bladder. The urinary bladder was so-identified by tracking the opisthonephric ducts from the trunk kidney to their termination within the bladder lumen. The specializations included a pronounced thickening of the bladder wall, an elaboration of the internal surface area by means of extensive septal intrusions, and the frequent presence of vesicular or colloidal secretions within the bladder lumen. The extensive septal intrusions as well as the inner wall of the bladder lumen were lined by a conspicuous tall columnar epithelium. The epithelium and septa were most prominent in fish having no or only small amounts of accumulated lumenal secretions. In other fish having large amounts of stored lumenal secretions, the septa and septal epithelia were reduced or absent, suggesting a holocrine form of secretion. The tall columnar epithelium in the bladder consisted of cells having granular cytoplasm and, in some instances, small amounts of contained material identical in appearance to the accumulated secretions occupying the bladder, suggesting that the epithelial cells may be the source of the secretions. Considerable amounts of stored secretions were present in approximately half of the specimens sampled from these two species. These secretions were resistant to digestion and clearing in counterstained skeletal preparations and were PAS-unreactive, suggesting the absence of a carbohydrate component. The composition and function of the secretions is currently unknown; however, the equal frequency of occurrence of secretions in immatures and adults of both sexes rules out a solely reproduction-associated function.