The amount of air normally trapped in the nonwettable fur of the muskrat has been measured. The average volume of air for 10 muskrats was 176. 9 ml, which represents 21. 5 per cent of their average total dry volume. The large volume of air in the fur gives the muskrats an average specific gravity of O. 790. The importance of the air layer for the buoyancy and insulation of the muskrat is discussed.