Optical hole burning has been observed in the 5941-Å transition of Pr3+ in a charge-compensated tetragonal site of CaF2. The Pr3+ ground state is doubly degenerate and shows a large first-order hyperfine splitting, which is clearly resolved because of the very narrow inhomogeneous linewidth of 650 MHz. The hole burning involves a new mechanism, in which optically induced spin flips of neighboring nuclei (here 19F) shift the optical transition frequency outside its homogeneous linewidth. This mechanism was confirmed by optical rf double resonance. © 1981 Optical Society of America.