Distinguish three basic positions of the mouth of fish: upper, terminal, lower. There are also transitional forms - mouth half upper, half lower. The position and structure of the mouth of fish depends on the nature of its diet.

Terminal mouth - both jaws are of equal length, with the mouth lying on the body axis. Fish with terminal mouth position prefer to grab prey that is directly in front of them. They are mostly predators.

Fish such as pike, perch, roach, roach, omul, pelada, grayling, nelma, herring, etc. have the final shape of the mouth.