Extracts from the leaves of plants infected with a wide range of viruses were stained with potassium phosphotungstate and examined in the electron microscope in an attempt to detect virus particles. Most specimens were made by applying a strip of tissue, peeled from the underside of a leaf, to a drop of stain on an electron microscope grid. The rod-shaped particles of barley stripe mosaic virus, tobacco rattle virus, tobacco mosaic virus, potato virus X, potato virus M, and potato virus Y could be detected easily. Measurement showed that, as in the shadow-cast preparations of other workers, the lengths of the great majority of particles of each virus fell within one, or with tobacco rattle virus within two, narrow ranges. For each virus the mean length of the particles within such a range was usually closely similar to the value obtained by others using shadowed preparations. Of the viruses with isometric particles, turnip yellow mosaic, wild cucumber mosaic, turnip crinkle, and broad bean mottle virus particles were usually easy to detect, principally because of the large numbers present. In preparations of turnip yellow mosaic virus round particles, larger than those of the virus and usually “empty” were occasionally present. Somewhat similar large “empty” particles were common in preparations of wild cucumber mosaic virus and frequently such a large particle enclosed an “empty” virus particle. Careful examination was necessary to detect particles of the viruses of cucumber mosaic, lucerne mosaic, tobacco necrosis, and tobacco ringspot, either because few particles were present or because they were poorly contrasted against the background.