1. Cellular morphogenesis during postembryonic brain development inDanaus plexippus plexippus L. was examined using histological techniques including radioautography. 2. The production of new neurones is continuous throughout larval and pupal stages and shows no fluctuations corresponding to ecdysis. Glial cell production, on the other hand, occurs at the time of molting. 3. New ganglion cells are formed by the division of neuroblasts found in aggregates or isolated among larval ganglion cells. Asymmetrical neuroblast divisions yield one neuroblast and one ganglion-mother cell which then divides at least once to form the new ganglion cells. Such divisions begin earlier inDanaus than in other investigated Lepidoptera. Symmetrical divisions yielding two neuroblasts also occur, but only among aggregated neuroblasts. 4. Radioautographs of brains fixed at progressive intervals after Tritiated Thymidine (H3TdR) injection have permitted description of the basic pattern by which cells of the adult brain cortex are laid out and progressive changes in the relationship of new ganglion cells derived from a single neuroblast. Ganglion-mother cells are deposited between the neuroblast and the neuropile, thus forming a row of cells which move the neuroblast progressively farther from the neuropile. New ganglion cells produced by ganglion-mother cell mitoses, which usually are oriented at 45° angles to the neuropile, expand the cell cluster. Differentiating fibers of these cells are apparent within a few days of their production and seem to enter the neuropile in one bundle. Later with increased neuropile volume and further cell differentiation the cells are no longer clumped and thus are not recognizable as offspring of a single neuroblast. 5. Neuroblasts found scattered among the larval ganglion cells arise from cells near the neuropile. These cells, at first indistinguishable from their neighbors, gradually assume the size and ready stainability of neuroblasts and subsequently divide according to the pattern described above. 6. Scattered neuroblasts degenerate beginning shortly after pupation and have completely disappeared by the end of the fourth day. 7. Except in the developing optic lobe, glial cell numbers increase through the proliferation of already existing glial cells. All glial cells show H3TdR uptake during a 12 hour period surrounding each larval-larval molt and for a somewhat longer period after pupation. However, in the larval stages mitotic figures were seen only among glial I, II, and IV. Glial I cells divide through the entire last larval stage and for two days following pupation. Large irregular mitoses seen among glial III cells at pupation indicate that these cells are probably polyploid. 8. In the newly forming adult optic lobe glial II, III, and IV cells appear to develop from preganglion cells or cells indistinguishable from them. These cells gradually stain more and more darkly, segregate into the normal glial positions, and subsequently divide in accord with other glial cells. 9. At the end of the fifth instar the perineurium (glial I cells), which begins to thicken during the third larval instar, is multilayered and contains many vacuolar cells. Just prior to pupation the neurilemma begins to disintegrate and during the next five days all but the cells closest to the brain disappear. Hemocytes are seen to engulf portions of the disintegrating neurilemma and already degenerating perineurial cells, but do not seem to engulf live cells. The glial I cells remaining adjacent to the brain secrete a new neurilemma. 10. There is no evidence for mass destruction of larval ganglion cells by either autolysis or phagocytosis, and only in the antennal center is there evidence of degeneration of larval cells (NORDLANDER andEDWARDS, in press).