Ultrastructural studies indicate that riboflavin deficiency induced by either dietary restrictions alone or with the addition of the antagonist galactoflavin severely affects the structural integrity of myelin lamellae. The degenerative process induced by riboflavin deficiency is time dependent. Nonmyelinated nerve fibers are not affected ultrastructurally by the deficiency. Cellular organelles of both myelinated and nonmyelinated nerve fibers remain intact and presumably functional.