Conditions on the electric field of an electromagnetic wave in a plasma are found which make the wave magnetic field comparable in strength to the ambient field. The conditions arise from higher-order forces than are used in linear theory. It is found that the strong 100-Hz signals observed at night by the Pioneer Venus orbiter in the Venus ionosphere at 129 km and a large fraction of the signals observed at 160 km, if interpreted as whistlers, would meet these conditions. The observations would represent highly nonlinear states for whistlers. Previous analyses of the 100-Hz signal interpreted as due to linear whistlers could therefore be flawed. This supports our contention [Cole and Hoegy, 1996a, b>, arrived at by an independent argument, that the whistler hypothesis for the 100-Hz signal is invalid. In addition, in a new contribution to linear theory of the dispersion relationship, it is found that in the Venus ionosphere, in the high collision frequency region near 130 km altitude, linear whistlers would propagate freely in any direction, contrary to assumptions otherwise in the literature. They are critically damped there. At 160 km, normal linear-limit theory applies and manifests normal guidance by the ambient magnetic field, subject to &ohgr;

cos &thgr;, with little damping.