The in vitro decapsidation of a small isometric plant virus, turnip yellow mosaic virus (TYMV), was investigated by cryo-electron microscopy. Cryo-electron micrographs of TYMV and empty shells show that rapidly frozen virions still contain their RNA. Images of vitrified virions resemble closely those previously obtained by negative staining. Rapidly frozen virions decapsidate upon thawing although they remain well dispersed on the grid. The escape of the RNA through a hole at the periphery of the capsid could be visualized. The results suggest a model for the in situ decapsidation of small icosahedral viruses.