  We report on the results of the longest uninterrupted observation of the
Anomalous X-ray Pulsar 1RXS J1708-4009 obtained with the BeppoSAX satellite in
August 2001. In the phase-resolved spectrum, corresponding to the rising part
of the pulse, we found an absorption-like feature at 8.1 keV (significance
level of 4 sigma), most likely due to cyclotron resonant scattering. The
centroid energy converts to a magnetic field of 9x10^{11} G and 1.6x10^{15} G
in the case of electrons and protons, respectively. If confirmed, this would be
the first detection of a cyclotron feature in the spectrum of an anomalous
X-ray pulsar. We discuss here the results also in ligh of recent theoretical
models for these class of sources.
