Osmoregulatory mechanisms were examined in major groups of fishes including hagfish, holocephalans, elasmobranchs, the coelacanth, and ray-fin fishes. Four main patterns of body fluid composition emerged. These represent the three main osmoregulatory processes in the marine environment and the one in fresh water. Some possible interrelationships among these four types are discussed. Urea retention in marine elasmobranchs and the coelacanth, although similar, may have been acquired independently during evolution. The importance of gills and the rectal gland in elasmobranch osmoregulation is discussed. The importance of amino acids in intracellular osmoregulation in elasmobranchs is also reviewed. Recent studies on water and electrolyte regulation in freshwater stingrays are summarized.