In our study of the extended supernova remnant G54.4–0.3, we find a CO emission shell at about 40km s−1 radial velocity, in striking alignment with the radio continuum shell. The correspondence in position and size argues strongly for a physical connection, and allows derivation of the kinematic distance as well as the mass and density of the CO shell. The correlation with an OB association supports the value derived for the kinematic distance. Model calculations suggest that the formation of the CO shell results from a stellar wind and that the supernova remnant has expanded into the wind bubble located inside the shell.