We demonstrate a new mechanism for optical spectral hole burning in which population is stored in electronic Zeeman sublevels. This is shown by optical pumping the ground-state Kramers doublet of the I9/242H11/2 (6248.9 AI) and I9/244G5/2 (5777.1 AI) transitions of LaF3:Nd3+ with a narrow-band cw dye laser. Experiments were carried out at 1.6 K in external fields of 100 G. The Zeeman shift of the resulting antiholes and holes makes an assignment of these structures possible. Hole recovery times of hundreds of milliseconds are shorter than expected at these low temperatures and fields and provide evidence for optical spectral diffusion caused by mutual spin flips in the ground state. © 1987 The American Physical Society.