The method by which the weight, volume, density and sinking factor of living aquatic organisms can be determined has been extended. By sinking factor is meant the ratio of the density of the organism to that of its environment at the time of the actual determination. The density of the environment is always raised to 1000, and the density of the organisms computed on this basis.A quick and accurate method for determining the percentage of water in a living organism has been described and discussed.On the assumption that a sponge consists of protoplasm and spicules only, the density of the protoplasm has been computed.The sinking factor of several fish and of the embryos of some Crustacea at different stages has been determined. One effect of the swim-bladder in teleost fishes is to reduce the sinking factor and thus facilitate swimming.The effect of the sinking factor on swimming in general and the effects of spinous outgrowths in pelagic Crustacea are discussed.