A nuclear gene mutation in Oenothera hookeri increases the frequency of variegated sectors. The gene is recessive; the variegation is cytoplasmically transmitted. Once variegation is induced, the mutant gene is not required for its continued expression. The induced sectors may differ one from another. The gene expresses unique patterns of penetrance and of maternal effect. The genetic data implicate the chloroplasts as the site for the expression of variegation. The chloroplasts of O. parviflora are also subject to the action of the nuclear gene. Possible mechanisms by which a gene might cause chloroplasts to mutate are discussed.