Spheroidal particles, 50-60 m¼ in diameter, were found in quick preparations and in ultra-thin sections for electron microscopy, from leaves of cassava plants infected with the vein mosaic (CVMV). They were not found in preparations made from healthy plants, nor in symptomless leaves of infected plants, and are considered as representing the virus. In ultra-thin sections of leaf tissues, CVMV particles were found dispersed in certain areas of the cytoplasm, rich in ribosomes, but poor in other cytoplasmic organelles, where sporadical small bundles of dense fibrils (7-10 m¼X 100-300 m¼) occur. Dense and fine granular masses (0.3-3 ¼,in diameter) were seen in a few instances, associated with the particles. It seems likely that both fibrils and the dense masses are primarily involved in the CVMV synthesis.