Although fish and reptiles share some characteristics, fish are not reptiles. The most important difference between the two groups of animals is that fish use gills to breathe, while reptiles use lungs.

Fish and reptiles are both vertebrates, which means that they have backbones, and most species are cold-blooded. Reptiles live on land and in water, whereas fish live exclusively in water. In addition, all reptiles have scales, but only some fish have scales. Reptiles include alligators, snakes, lizards and sea turtles. Fish include bony fish, such as salmon and trout; cartilaginous fish, such as sharks and skates; and lampreys and hagfishes.