  We have for the first time detected X-ray emission associated with a
protostellar jet, on the jet emanating from the L1551 IRS5 protostar. The IRS5
protostar is hidden behind a very large absorbing column density, making the
direct observation of the jet's emission possible. The observed X-ray emission
is likely associated with the shock ``working surface'', i.e. the interface
between the jet and the circumstellar medium. The X-ray luminosity emanating
from the jet is moderate, at $L_{\rm X} \simeq 3 \times 10^{29}$ erg/s, a
significant fraction of the luminosity normally associated with the coronal
emission from young stars. The spectrum of the X-ray emission is compatible
with thermal emission from a hot plasma, with a temperature of $\simeq 0.5$ MK,
fully in line with the temperature expected (on the basis of the jet's
velocity) for the shock front produced by the jet hitting the circumstellar
medium.
