Recently, ultralow-frequency waves with large azimuthal wavenumber (large m) have been observed on similar L shells and with the same (or similar) frequencies as small-m field line resonances (FLRs). The large-m waves appeared to the west of the small-m FLRs and had westward phase propagation while the small-m FLRs had tailward phase propagation. We propose an extension to an earlier waveguide model to explain these observations. We suggest that small-m tailward propagating waveguide modes drive the small-m FLRs. Phase mixing within these FLRs allows the development of the nonlinear Kelvin-Helmholtz (K-H) instability near the resonant field lines. Phase-mixing scale lengths are limited by ionospheric dissipation, and we show that realistic ionospheric Pedersen conductivities result in the dominance of a single zero-frequency K-H wave in each small-m FLR region having m consistent with observation of the large-m disturbances. K-H growth rates are significant, but not large enough to disrupt the small-m FLRs. We propose that unstable ion distributions amplify the seed K-H waves as the ions drift westward. This leads to observable large-m drift waves at or beyond the westward limits of the small-m FLR regions.¿ 1997 American Geophysical Union