Congenital dyserythropoietic anemia type I was diagnosed in a Japanese patient. Electron microscopic examination of thin sections revealed the features of erythroblasts peculiar to CDA type I; namely "spongy appearance" of nucleus and "internuclear chromatin bridge" etc. However, the widening of nuclear pores, which has been reported as another peculiar feature of CDA type I erythroblasts, were not confirmed. Aberrations of erythroblast nuclear membrane which have not been hitherto noticed were as follows: The nuclear envelopes of erythroblasts frequently lacked heterochromatin application in wide areas where they often showed a marked invagination or evagination. This is associated with the invasion of cytoplasmic organelles into the nuclear territory and occasionally with the extrusion of the nucleolus into the cytoplasm. There were no nuclear pores on the invaginated or the evaginated portions of the nuclear envelope where the perinuclear cistern was compressed to lose the cisternal space, or widened to contain some membrane debris. The findings with thin sectionings were confirmed with freeze-fracturing. Both findings suggest that an aberration may reside in the erythroblast nuclear membrane itself, but not in nuclear pores.