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SUMMARYBarley yellow striate mosaic virus (BYSMV) [cryptogram: *[* : *[* :U/* : S,I/Au] was detected in the cytoplasm but not in the nucleus or other organelles of infected barley cells. Stiff bacilliform particles of about 45 × 33o nm, believed to be the mature virus particles, occurred as membrane-bound aggregates while slightly flexuous particles of similar length and 35 to 45 nm wide, possibly representing 'immature' virus particles, were seen dispersed within sac-like structures. Such particles had sometimes an electron-transparent end and were associated with spherical vesicles 25 to 6o nm in diam.Electron-dense intracytoplasmic masses of granular or finely fibrous material, here called viroplasm, were consistently found in connexion with BYSMV particles which were apparently formed by budding from them through membranes limiting the surrounding enclaves. These appeared to have derived from the endoplasmic reticulum and, occasionally, from the separation of the nuclear envelope lamellae. The number and size of BYSMV viroplasms, together with their close connexion with both the endoplasmic reticulum and the outer lamella of the nuclear envelope, seem to be features unique among the described plant rhabdoviruses.

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