Recent observations of two-photon absorption within the configuration 4f7 of Gd3+ in LaFs have indicated an anomalously high line strength for the transition 8S7,2+ 6P7,2 compared with .+ 6 P~,~ and *S7;2 -+ 6P3i2. It is shown that the standard expression for the two-photon intensity is unusually small for all three transitions and that a higher-order correction, involving matrix elements of the spin-orbit interaction within the intermediate configuration 4f65d, predominates. The principal term is scalar, implying (in the absence of J mixing by the crystal field) isotropic contributions to the various components of the AJ = 0 transition. The line strengths that have been observed can be accounted for reasonably well without having to invoke variations in the radial integral (4f lr15d) or inadequacies in the closure approximation. Expressions for the transition matrix elements are given for the general configuration iN.

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