  We describe some of the first X-ray detections of groups of galaxies at high
redshifts (z~0.4), based on the UK deep X-ray survey of McHardy et al (1998).
Combined with other deep ROSAT X-ray surveys with nearly complete optical
identifications, we investigate the X-ray evolution of these systems. We find
no evidence for evolution of the X-ray luminosity function up to z=0.5 at the
low luminosities of groups of galaxies and poor clusters (Lx>3e42 erg/s),
although the small sample size precludes very accurate measurements. This
result confirms and extends to lower luminosities current results based on
surveys at brighter X-ray fluxes. The evolution of the X-ray luminosity
function of these low luminosity systems is more sensitive to the thermal
history of the intra-group medium (IGM) than to cosmological parameters. Energy
injection into the IGM (from for example supernovae or AGN winds) is required
to explain the X-ray properties of nearby groups. The observed lack of
evolution suggests that the energy injection occured at redshifts z>0.5.
