Cytological changes in the ultrastructure of cambial cells of teak (Tectona grandis) during active and dormant periods have been studied after glutaraldehyde-osmium tetroxide fixation. The fusiform and ray initials are essentially alike in their ultrastructure and all have cellular organelles. Active cambial cells are highly vacuolated and are rich in all organelles. They contain rough endoplasmic reticulum (ER), polysomes, dictyosomes with hypertrophied cisternae, mitochondria and plastids with starch. The plastids are characterized by the presence of varying amounts of a dark intralamellar inclusion which is considered to be a protein. Microfilament bundles, 0.05 to 0.2 üm in width, occur frequently in the parietal cytoplasm. Dormant cambial cells have numerous small vacuoles, smooth ER, free ribosomes and a few inactive dictyosomes. In addition to the dark inclusion, plastids contain what are thought to be phytoferritin granules and prolamellar bodies. Branched plasmodesmata and paramural bodies are associated with the cambial cell walls.