  We report 209Bi NMR and NQR measurements on a single crystal of the Kondo
insulator U3Bi4Ni3. The 209Bi nuclear spin-lattice relaxation rate ($T_1^{-1}$)
shows activated behavior and is well-fit by a spin gap of 220 K. The 209Bi
Knight shift (K) exhibits a strong temperature dependence arising from 5f
electrons, in which K is negative at high temperatures and increases as the
temperature is lowered. Below 50 K, K shows a broad maximum and decreases
slightly upon further cooling. Our data provide insight into the evolution of
the hyperfine fields in a fully gapped Kondo insulator based on 5f electron
hybridization.
