  Combining old and new de Haas-van Alphen (dHvA) and magnetoresistance data,
we arrive at a detailed picture of the Fermi surface of the heavy fermion
superconductor UPt3. Our work was partially motivated by a new proposal that
two 5f valence electrons per formula unit in UPt3 are localized by correlation
effects -- agreement with previous dHvA measurements of the Fermi surface was
invoked in its support. Comprehensive comparison with our new observations
shows that this 'partially localized' model fails to predict the existence of a
major sheet of the Fermi surface, and is therefore less compatible with
experiment than the originally proposed 'fully itinerant' model of the
electronic structure of UPt3. In support of this conclusion, we offer a more
complete analysis of the fully itinerant band structure calculation, where we
find a number of previously unrecognized extremal orbits on the Fermi surface.
