Abstract Leaves of Chenopodium amaranticolor were inoculated either with large amounts of tobacco mosaic virus (TMV) to produce massive infections or with less, to yield separated lesions. Sections of massively infected leaves were examined by electron microscopy at intervals up to 72 hours following inoculation; lesions were similarly studied from first appearance (at about 48 hours) until they were 10 days old. In massive infections, virus particles were first seen 30 hours after inoculation though no necrosis was apparent until 15 hours later. In older infections, great numbers of particles in crystalline arrays were found in about one-third of the cells, but interspersed with these were other cells in which little or no virus was detected. The virus appeared only in the cytoplasm. In cells which contained virus but were not yet necrotic, nuclei appeared to be unaffected. Regions containing filamentous forms, very like those associated with TMV infections in tomato and tobacco, were found in virus-containing cells, and the first appearance of filaments coincided with that of virus. In isolated lesions the pattern of virus distribution was similar; virus-containing cells were found throughout the necrotic area and for a distance of five to ten cells radially beyond. Thus in most respects the early progress of TMV infection in C. amaranticolor and in Turkish tobacco seem closely parallel. The necrosis that occurs in Chenopodium is an event that takes place only after virus has been synthesized.