  We report the detection of extended X-ray emission around several powerful
3CR quasars with redshifts out to 0.73. The ROSAT HRI images of the quasars
have been corrected for spacecraft wobble and compared with an empirical
point-spread function. All the quasars examined show excess emission at radii
of 15 arcsec and more; the evidence being strong for the more distant objects
and weak only for the two nearest ones, which are known from other wavelengths
not to lie in strongly clustered environments. The spatial profiles of the
extended component is consistent with thermal emission from the intracluster
medium of moderately rich host clusters to the quasars. The total luminosities
of the clusters are in the range 4x10^44 - 3x10^45 erg/s, assuming a
temperature of 4keV. The inner regions of the intracluster medium are, in all
cases, dense enough to be part of a cooling flow.
