We report the first ESR signal of an excited crystalline state (Γ6) in a metal (PdEr). The intensity of the Γ6 isotropic resonance compared to the Γ8(3) resonances of the ground-state quarter allows the determination of the energy scaling parameter W of the crystalline field. A more precise value is deduced from the splitting of the transitions 1 → 2 and 3 → 4 of the Γ8(3) ground state: W=-0.163±0.015 K, in accordance with the work of Praddaude.