The onset of heavy-fermion coherent-ground-state behavior is studied in CeCu6 by Hall-effect and magnetoresistance measurements. CeCu6 is an ideal system for this study since the fermions are extremely heavy and the system does not become either magnetic or superconducting. The strong temperature dependence of the Hall effect sets the scale for the high-temperature, single-Kondo-impurity, incoherent regime, a broad transition region, and the very-low-temperature coherent regime. In contrast to the resistivity, which shows a gradual transition to the coherent state, the Hall effect shows a rather sharp feature at the onset of coherence. © 1986 The American Physical Society.