  For the first time in the history of high energy astronomy, a large CdTe
gamma-ray camera is operating in space. ISGRI is the low-energy camera of the
IBIS telescope on board the INTEGRAL satellite. This paper details its design
and its in-flight behavior and performances. Having a sensitive area of 2621
cm$^2$ with a spatial resolution of 4.6 mm, a low threshold around 12 keV and
an energy resolution of $\sim$ 8% at 60 keV, ISGRI shows absolutely no signs of
degradation after 9 months in orbit. All aspects of its in-flight behavior and
scientific performance are fully nominal, and in particular the observed
background level confirms the expected sensitivity of 1 milliCrab for a 10$^6$s
observation.
