By employing Haselgrove equations and a diffusive equilibrium model for the electron-ion density distribution the the ionosphere, ray-tracing computations are carried out to study the propagation characteristics of whistler waves observed in the low-latitude ground stations. It is shown that such waves are propagated in the ionosphere in the prolongitudinal mode of propagation under the influence of suitable negative latitudinal gradients of ionization. A positive correlation between the high whistler activity at the ground and the negative latitudinal gradients of ionization existing in the ionosphere at low latitudes during the time of observation is found. This result explains satisfactorily the problem of how the whistlers are observed on the ground at low latitudes in the absence of suitable ducts in the ionosphere.