Several thousand impulsive whistler mode noise bursts were detected by the Pioneer Venus wave instrument during the first 10 seasons with nightside traversals at low altitudes. The altitude distribution for these events shows that essentially all of the bursts were detected when the orbiter was less than 2000 km above the planet, suggesting that the varying plasma conditions could not maintain coherent whistler mode field-aligned guidance over greater distances. Within the 2000-km range, the distribution of the number of events (N) versus altitude (h) shows that there are two distinct subregions: for h<800 km we find N varying as (h-h0)-2, 0≤h0≤50 km, and for 800<h<2000 km we find N approximately constant, with significant fluctuations that are frequently correlated with passage over surface features. These results are interpreted in terms of two types of whistler mode propagation from sources below the ionosphere; we consider direct detection of near-field radiation for h<800 km, and detection of ducted whistlers for the 800<h<2000 km range. ¿ American Geophysical Union 1987