Leaves of Chenopodium amaranticolor were infected with sowbane mosaic virus (SMV), sectioned and examined by electron microscopy. Leaves of Brassica pekinensis and C. amaranticolor were infected with turnip yellow mosaic virus and cowpea mosaic virus, respectively, and similarly studied. With all three viruses it was difficult, in sections, to distinguish the small isometric virus particles from ribosomes though sometimes this was possible, especially when the viruses crystallized. Pretreatment of tissue with permanganate or EDTA appeared to destroy the ribosomes but resulted in excessive disorganization of the tissue. Although SMV did not normally crystallize, wilting the infected leaves caused it to do so. All three viruses were found free in the cytoplasm and were absent from nuclei, chloroplasts, and mitochondria. Some abnormal structures found in the infected tissues are described.