  The frequency-dependent conductivity $\sigma_{xx}(\omega)$ of 2D electrons
subjected to a transverse magnetic field and smooth disorder is calculated. The
interplay of Landau quantization and disorder scattering gives rise to an
oscillatory structure that survives in the high-temperature limit. The relation
to recent experiments on photoconductivity by Zudov {\it et al.} and Mani {\it
et al.} is discussed.
