The occurrence of at least one lipase may be assumed in all fishes and has been demonstrated for a number of species. In carp and killifish extracts of intestine showed lipolytic activity. In goldfish, lipase activity occurred in extracts of a mixture of liver and pancreas and in the intestinal contents. Esterase (another lipase) activity has been found in the liver, spleen, bile, intestine, pyloric caecae and stomach of rainbow trout. Use of radioisotope-labeled lipids in cod suggested that the cod's lipase acted in the same manner as mammalian pancreatic lipase, although it was not considered more than a suggestion that fish lipase is of pancreatic origin. Regardless of origin, some kind of lipase is essential to fish because fatty acids are essential dietary components for fish.