The inertial range of weak MHD turbulence in the interstellar medium is calculated. The spectrum in the inertial range is derived from wave–wave interaction formulae. The interstellar medium is considered as consisting of four types of regions: Clouds, Cool Intercloud regions, Hot Intercloud regions, and Tunnels. The damping of the turbulence is calculated for four mechanisms: ion–neutral collisions, thermal conductivity, viscosity, and resistivity. Ion-neutral collisions are found to be the dominant damping mechanism in the Clouds and Cool Intercloud regions, but may be insufficient to terminate the inertial range in the Hot Intercloud regions. Thermal conductivity is so high in the Tunnels that no inertial range is expected. The longest inertial range is expected to be in the Hot Intercloud regions, but the length is very dependent on the temperature and ionization structure of those regions.