Nuclear polarization of 169Tm(I=12) is achieved in a crystal of CaF2 containing a small fraction of paramagnetic Tm2+ ions (S=12) by optically pumping with circularly polarized light in the region 5400-6000 Å, where there is a large magnetic circular dichroism. By optical pumping alone at T=1.65 ° K and in a field H=750 G a nuclear polarization of 9% is observed. If the I+S- rf transition is simultaneously saturated, the polarization increases to 18%. The polarization is reversed by changing from left- to right-circularly polarized light. These results are understood in terms of a model in which a large degree (~90%) of nuclear-spin memory exists in the optical-pumping cycle. The experiment proves the feasibility of significant nuclear polarizations in solids by optical pumping. A small polarization of the abundant 19F nuclei is produced by a three-spin cross-relaxation process with two optically pumped 169Tm2+ ions.