  We present X-ray observations of the afterglow of GRB000926 performed around
and after the break observed in the optical light curve two days after the
burst. The steep X-ray light curve observed around the break confirms the
presence of this feature in X-rays. However, the spectral and temporal
properties are not consistent with a standard jet scenario based on synchrotron
emission, requiring a more complicated model. We find that X-ray and optical
data are compatible with a moderately collimated fireball (with opening angle
$\theta\approx25 \deg$) expanding in a dense medium ($n\approx 4\times 10^4$
cm$^{-3}$). This produces two breaks in the light curve. The first, at
$t\approx$ 2 days, is due to jet behavior. The second, around 5 days, is
attributed to the transition of the fireball to a non-relativistic expansion.
This transition predicts a{\it flattening} of the light curve, that explains
the late X-ray measurement in excess above the extrapolation of the simple jet
scenario, and is also consistent with optical data.
