Analyses were made of the gases of swimbladders from 37 spp. (384 specimens) of marine fishes. On the reasonable assumption that N2 cannot be chemically secreted we would presume that the N2 tension in the swimbladder at a given depth would in time reach equilibrium with N2 tension in the surrounding seawater. As the latter is near 0.8 atm. at all depths it follows that the N2 percentage in the swimbladder should be approx. 80 divided by the total pressure in atmospheres. This proved essentially correct for most spp. In some the correlation fails completely and the N2 percentage at all depths may be as high as in surface fishes, i.e., about 80%. By measuring in a pressure tank the depths at which these fishes just float it was always found to be near the surface, and they had therefore not secreted O2 while in deep water. In most cases these fishes were known to be vertical migrators. It is hence believed that a greatly elevated N2 percentage in the swimbladder as compared with the depth is indicative of vertical migration. CO2 tension in all cases was found to be very low, so that practically the total gas pressure in the swimbladder above 0.8 atmospheres is made up of O2.