An electron microscope study of different hosts infected with turnip crinkle (TCV) and saguaro cactus (SCV) viruses, two tentative members of the tombusvirus group, was carried out. Particles of both viruses were readily detected in cells of different tissues, in the cytoplasm of which they occurred in great numbers, though not in crystalline arrays. Cytological modifications of various types were also observed. The most striking of these was an extensive peripheral vesiculation of mitochondria in TCV-infected cells, which was accompanied by plastic activity of the organelles that often engulfed portions of ground cytoplasm and virus particles. Cytoplasmic inclusion bodies were not present. Likewise, none of the cytopathological features characterizing tombusvirus infections was observed. No indications were obtained to support the idea that TCV and SCV may continue to be considered members, even though tentative, of the tombusvirus group.