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The hyperfine splitting of holmium, as a dilute (1%) substituent in ferromagnetic GdAI2, has beenstudied by spin-echo NMR at liquid helium temperatures. The measurements were made on oriented single crystals. with fields up to 8 T applied along twofold, threefold and fourfold crystallographic symmetry axes. The field dependence of the hyperfine splitting is interpreted in terms of the standard three-parameter mean-field model for lanthanide ions at sites of cubic symmetry. The crystal-field coefficients are found to be 30% to 40% smaller than those for pure HoAI?; moreover, the molecular field seen by the holmium ion is more than 20% below the lower bound derived.from a current model of exchange in RAlz compounds. These unexpected results are strikingly confirmed by a recent analysis of FMR data on a dilute H o : GdAI2 alloy. Possible explanations are considered.