Using source material isolated from the thermal neutron fission of 235U, an atomic beam investigation of 138Cs has been carried out. The results are I = 3 and [Formula: see text] nuclear magnetons. Calculations using a delta-function residual n–p force show that no pure shell-model configuration, appropriate to the mass, is capable of yielding the observed spin. Although the magnetic moment is just consistent with either (g7/2)7/2 or (g7/2)5/2 for the proton configuration and ƒ7/2 for the odd neutron, one may conclude that some configuration mixing is involved. The proton part of a plausible wave function is predominantly (g7/2)7/2 with a spin-5/2 admixture—either (g7/2)5/2 or (d5/2)5/2. In this respect it is similar to the wave function of 140La.