Ultrastructural studies indicate that galactoflavin-induced riboflavin deficiency induces asynchrony of rat erythroblast maturation. During the latter stages of maturation erythroblasts retain significantly larger numbers of ribosomes as compared to control cells. Nucleoli are not evident in erythroblasts whose nuclei indicate cells in the latter stages of development. Membrane whorls develop within the mitochondria of plasma cells, eosinophils and neutrophils during the fifth week of riboflavin deficiency. No further evidence of degeneration was noted among additional cell organelles.