The speed of longitudinal sound waves in Antarctic krill has been measured by the time?of?flight method. The result of 17 separate measurement series on different assemblages of living krill is that the animal’s sound speed exceeds that of seawater at the same temperature by 2.79±0.24%. The mean lengths vary from 29.4 to 38.9 mm, with overall mean 32.2 and s.d. 2.5 mm. The corresponding density of krill of mean length 31 mm is 1.0647±0.0069 g/cm3 . Measurement temperatures varied from 5.3 to 12.1°C; corresponding salinities varied from 32.5 to 33.87 ppt, which also represent the ambient state. The ambient sea temperature was 2.0±0.3°C.