Publisher Summary This chapter examines the morphology and function of the pseudobranch. Teleostean pseudobranch is a hemibranch and is maximally developed in highly evolved fish. In the teleosts, the pseudobranch is located in the cranial part of the subopercular cavity to which it is attached by one of its sides. Histological serial sections and cast preparations demonstrate that the pseudobranch has an arterio-arterial vasculature similar to that of the gill. The afferent pseudobranchial artery gives rise to successive arteries supplying filaments. Different types of pseudobranch organization have been observed according to the fresh- or saltwater origin of the fish, but it has been noticed that forms, sizes, and external structures vary widely within orders, families, and species without any obvious reason, whereas vascular organization and innervation keep a similar organization throughout. In some part of the pseudobranch with free lamellae, the epithelium consists of typical chloride cells in place of pseudobranchial cells. In the pseudobranch, chloride cells exhibit the characteristic differences of fresh- and saltwater species.