  We show that roton-like excitations are thermally induced in a
two-dimensional dilute Bose gas as a consequence of the strong phase
fluctuations in two dimensions. At low momentum, the roton-like excitations
lead for small enough temperatures to an anomalous phonon spectrum with a
temperature dependent exponent reminiscent of the Kosterlitz-Thouless
transition. Despite the anomalous form of the energy spectrum, it is shown that
the corresponding effective theory of vortices describes the usual
Kosterlitz-Thouless transition. The possible existence of an anomalous normal
state in a small temperature interval is also discussed.
