ABSTRACT The mean sinking factor of each of eight species of Cypriniformes has been determined by a method involving measurement of the pressure at which anaesthetized specimens float. It was found to lie between 1001 and 1004 for all species except the gudgeon, for which it is 1008. Roach, dace and carp have swimbladdere of very different sizes. It was shown that differences in the weight and density of the skeleton and scales, and in muscle density, are important factors in determining the volume of swimbladder gas necessary for neutral buoyancy. Differences in fat content are unimportant in these fish.