A set of organs (kidneys, ureters) responsible for excretion of end products of metabolism, excess water, salt, harmful and poisonous substances, and regulation of the internal environment. In the embryonic state in all fish, some fish in the adult state (mullet, beluga, gobies) have a functioning pronephric (head) kidney. Most fish have mesonephric (trunk) paired kidneys with ureters. In bony fish, the ureters in their terminal part form a bladder that opens outward near the genital opening.

Unlike higher vertebrates, which have a compact pelvic kidney (metanephros), fish have a more primitive trunk kidney (mesonephros) and their embryos have a kidney (pronephros). In some species (goby, aterine, beldyuga, mullet) the kidney in one form or another performs an excretory function in the adult; in most adult fish the mesonephros becomes a functioning kidney.