The damping of proton whistlers is found to be much more gradual than predicted by the cyclotron damping theory of Gurnett and Brice (1966) for an assumed homogeneous plasma with an isotropic Maxwellian proton velocity distribution. Several possible causes of the discrepancy are: nonlinear effects, anisotropy, a non‐Maxwellian velocity distribution, and irregularities in proton density. It is concluded that plasma density irregularities are responsible for the observed gradual damping. When these effects are included, the deduced proton temperature increases significantly (about a factor of 2) and the magnitudes of the variations in proton plasma frequency estimated are about 15–