We present a new spherical harmonic expansion of the global hotspot distribution, with the contribution of each hotspot weighted by its buoyancy flux. The resulting power spectrum, normalized to that expected for a random distribution, has statistically significant peaks only at degrees 1 and 2. To explain this, we study a simple model for the origin of mantle plumes in the D' layer at the base of the mantle: the Rayleigh-Taylor instability of two superposed fluid layers with different viscosities and densities in a spherical shell. The wavelength of the fastest growing instability greatly exceeds the thickness of D' and increases as the viscosity contrast &ggr; across this layer increases. These results are consistent with the hotspot spectrum if &ggr;~106. ¿ American Geophysical Union 1994