The fixed nucleus of Vicia faba has been investigated in squash preparations and in sections and it has been shown to consist of separate regions: the peripheral zone, in which lie the chromosomes; the middle zone, which is a protein region, the size of which varies with the mitotic activity of the root and in which lie the nucleoli; the nucleoli, each of which contains an inner core of protein material surrounded by an outer zone of material which probably contains nucleoprotein. The nucleolus also bears the nucleolar organizer, which is connected to the peripheral zone by a fine chromosomal strand. It is suggested that the apparent expansion of the middle zone during the period of maximum mitotic activity of the root is due to hydration of the protein. Similar zonation of the nucleus and nucleolus has been found in fixed root tips of Allium Cepa and Zea mays. Nuclei of living staminal hair cells of Tradescantia bracteata have been studied with ultraviolet microscopy and have been shown to possess the same structure as was found in the fixed nuclei of Vicia faba. This suggests that, at least in these cells, the middle zone and nucleolar core are not artifacts.