Comparative electron microscopy of tissue sections of different plants, infected with one of the 17 different strains of potato virus Y (PVY) found in São Paulo state was carried out. Infection of plants, with any of the strains utilized, was always followed by the appearance of characteristic inclusions in the cytoplasm. These inclusions appeared as linear elements (6-7 m¼ thick) arranged in a loose parallel array, or forming curled, ring- or pin-wheel-like structures. Similar inclusions are described in tissues infected with other viruses morphologically similar to PVY. The role of these structures in the virus infection is still not known. Occasionally, particles resembling those found in vitroappeared scattered randomly in the cytoplasm or sometimes associated with the cylindrical inclusions. Infection with strains Yc and Yel resulted in occasional appearance of aggregates of thick and elongated particles, abt- 50-60 m¼ in diameter. On the other hand, in tissues infected with strains Yp and Ynt, membraneous structures were found conspicuously in the cytoplasm, which consisted of a pile of flat cisternae. It seems that these structures might represent hypertrophied Golgi elements.