For small fluctuations of a uniform supersonic gas jet about the state in which it enters a medium with zero excess-pressure it is possible to express the ‘wave-length’ in terms of the mean velocity of the jet. The mean velocity is that on the bounding stream-lines and not the velocity of efflux as assumed by Prandtl in an earlier investigation. With this correction, better agreement is obtained between theoretical and experimental values for the wave-length of cylindrical jets. It is shown that the approximate theory may be expected to have a wider range of application to jets with axial symmetry than to plane jets, and to forecast wave-lengths for pressure ratios outside the range for which it may be applied to give conditions inside a jet.