An experimental study of sound scattering by a thin-walled cylindrical shell stiffened by an internal lengthwise rib is presented. The results show that, in the explored frequency band, the positions of the resonance frequencies of the wave S0 are not affected much by the presence of a stiffener. Measurements made during the transient state after the quasiharmonic excitation of the stiffened shell have shown that, at the S0 wave resonance frequencies, backscattering is at its maximum when the rib is located at the vibration node of the shell. On the other hand, when in the total backscattered pressure, forced vibration is also taken into account, the maxima are detected when the rib is located at the vibration antinodes. In the stiffened shell other additional resonances appear which are not excited in the case of an unstiffened shell. These resonances are attributed to the propagation of the flexural type waves which are not detected in an unstiffened shell. In a stiffened shell these resonances appear as a result of the interaction of the rib and the shell at their structural joint. These additional resonances are better observed when the rib is located at the ‘‘illuminated’’ part of the shell which indicates that most of the energy is scattered by the rib and shell joint. In the case of a short-pulse excitation, specular reflections from a joint can be observed. The experimental results correlate very well with the theoretical models.