We have measured the real part of the diagonal conductivity, Re(sigma(
xx)) for a 2D hole system in the low Landau filling, insulating regime
. The spectra, Re(sigma(xx)) versus frequency f, show a well-defined,
single resonance peak, which is surprisingly sharp. Plots of the reson
ance peak frequency and integrated intensity versus the applied magnet
ic held B are in conflict with the model of a 2D harmonic oscillator i
n B. (C) 1998 Elsevier Science B.V. All rights reserved.