The propagation of the fast mode is considered in nonuniform waveguides. We show how the natural dispersion inherent in a waveguide will select waveguide modes with a small wavenumber (ky) along the guide to remain near a localized source region of fast mode energy. It is these modes that are shown to have a coherent periodic time dependence over many cycles that are suitable for driving observable Alfv¿n resonances (magnetic pulsations). We expect the frequencies of Alfv¿n resonances to be very close to the eigenfrequencies of waveguide modes with ky=0. ¿ American Geophysical Union 1994