The instability of a circular cylindrical jet of liquid in air is studied on the assumption that the wavenumberkof the disturbance is complex while its frequency&sgr;is real. This implies that the disturbance grows with distance along the jet, but that it does not grow with time. The occurence of such disturbances is called spatial instability, in contrast to the temporal instability studied by Rayleigh and others, in whichkis real and&sgr;is complex. It is found that there are infinitely many unstable modes for the axially symmetric case and also for each of the asymmetric cases. In the case of high velocity jets, one of these modes for the symmetric case corresponds to the mode Rayleigh found. However, it is not the most rapidly growing mode. Both analytical and numerical solutions of the dispersion equation are given forkas a function of&sgr;and of the dimensionless jet velocity.