1. Pressures on the body surface of bluefish, swimming at 0-6 m.p.h., compared to lateral pressure, were recorded using strain-gauge manometers. 2. While in motion, the front of the fish is subject to a head-on pressure exactly equal to that calculated using the Pitot equation. In salt water, this pressure, in cm H 2 O, is equal to the speed, in m.p.h., divided by 0.98, all squared. 3. On the widest diameter the pressure is negative while swimming. This is attributable to the Bernoulli effect. 4. Pressure on the base of the tail is still negative, but not so negative as on the shoulder, in live swimming fish. The body and tail motion seem to draw water away from the peduncle of the tail, thereby diminishing turbulence. 5. In a dead fish the pressure at the base of the tail is positive, suggesting the possibility of boundary-layer separation and increased drag in dead fish towed through water. 6. The hydrodynamic pressures in fish swimming are often as great as the hydrostatic pressures encountered in animals of equal length subjected to gravity. We conclude that body defences against hydrodynamic pressure would also be useful against gravitational hydrostatic pressure. 7. Body structures which appear to resist hydrodynamic pressures in water and hydrostatic pressures on land are the skull, the vertebral column, and the circulatory system. 8. Transition from aquatic to terrestrial life may have been facilitated by adaptation to the pressures encountered on the body surface while swimming.