SUMMARYA detailed fine structural study of the nondividing vegetative cell of the centric diatomMelosira variansreveals peripheral organization of cytoplasm and organelles around a central vacuole. Polarity of the cell is such that the nucleus is found at the valve face of the epitheca. The unusual shape of the nucleus and its associated microtubular system and polar body are thought to relate to the migration of the nucleus during other phases of cell growth. Unusual discontinuities of the plastid lamellae are reported and the significance of the periplastid reticulum is discussed. Possible roles for the cytoplasmic vesicles and the membrane system in the cell are also considered.