Ultrathin sections of oat, wheat, and ryegrass leaves from healthy plants and plants infected with rhabdoviruses by leafhoppers Laodelphax striatellus Fallen were studied under the electron microscope. The bacilliform virions often surrounded by endoplasmic reticulum (ER) membranes, viroplasm, and tubular structures conforming, in diameter and structure, to the rhabdovirion nucleocapsid were observed in the cytoplasm of leaf cells of the diseased plants. The cereal pseudorosette virus [(165-200) x (63-70) nm, CPV] is the causative agent of the disease of cereals in Siberia. The mycoplasma-like organisms were found in the phloem cells of plants infected with CPV. The cereal mosaic virus [(360-420) x (56-64) nm, CMV] is the causative agent of the disease of cereals in the Russian Far East. CMV appears to be a strain of the northern cereal mosaic virus occurring in Japan and China.