Previous work on the vibrations of infinitely long cylindrical shells submerged in an infinite acoustic medium is extended to the case where the medium is bounded by a plane surface, either rigid or free. The solution leads to an infinite set of simultaneous, linear, algebraic equations, the coefficents of which are obtainable in closed form. If this set is replaced by a finite number of equations, a procedure is given for estimating the accompanying error. Typical numerical examples indicate that significant results can be obtained, in certain cases at least, using only a small number of equations. For steel shells in water, the results show that the presence of a fluid boundary has surprisingly little effect on either the response of the shell, in the case of forced vibrations, or the frequencies of free vibration, even when the shell is in close proximity to the boundary.