We report the observation of the resonance of Er³⁺ in Pd single crystals. The ground state of the ion is found to be a Γ₈⁽³⁾ quartet. A recent measurement on Er³⁺ in polycrystalline palladium¹ suggests that the ion has a Γ₇ ground state, this discrepancy with the results reported here is discussed. From measurements of the g-shift and variation of the linewidth with temperature we determine the exchange coupling constant J_fs, to be −0.014 ± 0.002 eV. Measurements of the hyperfine splitting observed for the weak Er¹⁶⁷ isotope give a value of 64 ± 5 G.