Electromicroscopic observations of Tomato Spotted Wilt Virus (TSWV) were done using ultrathin sections of Nicotiana glutinosa sistematically infected. The observed material was fixed with Karnowsky solution and treated with Osmiun tetraoxide in order to increase the constrast.
Uranyl acetate and Pb citrate stainswere used. The section was made using glass knives in a Porter, Blum MTl microtome. ARCA EMU 3H electromicroscope, working at 50 KW was used.
Virus particles, were observed in the tissue. These particles are spherical, 80 mu in diameter, surrounded with a membrane. They were forming clusters inside of vasicles in the endoplasmic reticulum of thecell.

1 Ingeniero Agrónomo M. S. Profesor Cátedra Fitopatología General, Facultad de Agronomía, Universidad de Chile