[en] We develop a theory for the electronic excitations in UPt₃ which is based on the localization of two of the 5f electrons. The remaining f electron is delocalized and acquires a large effective mass by inducing intra-atomic excitations of the localized ones. The measured de Haas-van Alphen frequencies of the heavy quasiparticles are explained as well as their anisotropic heavy mass. A model calculation for a small cluster reveals why only the largest of the different 5f hopping matrix elements is operative, causing the electrons in other orbitals to localize