  The BL Lac object Mkn 501 was observed with the BeppoSAX satellite at three
epochs in April-May 1998, simultaneously with the Whipple and HEGRA Cherenkov
telescopes. The X-ray spectrum is well detected up to 70 keV and it exhibits,
at all epochs, a continuous curvature, which is here modeled with three
power-laws of increasingly steeper index at larger energies. In the nu*f_nu
representation the spectrum exhibits a peak at ~20 keV, which is interpreted as
the maximum of the synchrotron emission. This implies that the synchrotron peak
energy has lowered by an order of magnitude with respect to the powerful X-ray
outburst observed in April 1997. The simultaneous TeV flux was comparable to
the lowest levels observed for Mkn 501, possibly suggesting that the peak of
the inverse Compton radiation had also shifted toward lower energies.
