  We present new measurements of the thermal conductivity of UPt3 down to very
low temperatures (16mK) and under magnetic fields (up to 4 T) which cover all
the superconducting phases of UPt3. The measurements in zero field are compared
with recent theoretical predictions for the thermal conductivity, which is
dominated by impurity states at the lowest temperatures studied. The
measurements under magnetic field at low temperatures are surprising since they
don't show the expected low field square root dependence. The discontinuity of
d kappa/dT at Tc changes drastically when passing from the high field low
temperature C phase to the low field high temperature A phase : this is related
to the change of the symmetry of the superconducting order parameter when
crossing the A - C phase transition.
