The effects of channel walls on the hydrodynamic properties of a floating vertical cylinder are examined. An interior eigensolution under the cylinder is matched with an exterior eigensolution in a manner similar to Yeung [1]. Wave effects due to an image cylinder can be conveniently expressed in terms of the coordinates of the central cylinder by the use of Graff's theorem. The infinite array results in a slowly convergent series which has to be summed with caution. Results for the heave added mass and damping of a cylinder for several geometric configurations are obtained. Also presented in the paper are results for the diffraction of incident waves about the same cylinder. The channel walls exert an important influence on the radiation and diffraction properties, the latter to a lesser extent. Such influence is characterized by the presence of "spikes" at wave frequencies corresponding to the occurrence of symmetric transverse resonant modes in the channel. An analytical solution of a threedimensional flapper wavemaker given in the Appendix further confirms such characteristics. In the high-frequency range, the radiation properties approach those of a single cylinder. In the low-frequency limit, they exhibit a behavior similar to that of a two-dimensional horizontal cylinder heaving in water of finite depth.