Tubular particles, about 25 m¼ wide, with an axial channel 4 m¼ in diameter, were found in leaf dip or purified preparations and also within tissues, from plants infected with pepper ringspot virus (PRSV), but not in uninoculated control plants. The particle length determinations showed two prevalent types of particles in vitro, respectively with ca. 55 m¼ and 200 m¼ in normal length. These values were independent of the virus isolate, host plant or preparative procedures for electron microscopy employed. In thin section profiles, a dense rim was observed bordering the axial channel, in cross section of particles stained with uranyl acetate, which might represent the location of the viral nucleic acid in the virion. The long, 200 m¼, particles can be separated from the short, 55 m¼ particles through sucrose gradient density ultracentrifugation, when it can be demonstrated that only the longer particles are infective. Incubation of purified preparations with ribonuclease and/or trypsin, did not affect infectivity or the structure of the virions. However, in some experiments, the trypsin treatment induced an end-to-end aggregation of the particles. Purified preparations lose infectivity after being heated for 10 minutes at 65-70°C. Simultaneously a generalized degradation of virions is noticeable. The end product of this degradation is a powdery material, composed of granules 30-40 Aº in diameter. In ultrathin sections of PRSV-infected tissues, ordered aggregates of particles, similar to those found in vitro,were observed in the cytoplasm. Such inclusions consisted of long (200 m¼) particles only. In no instance, groups of typical 55 m¼, short particles were found, and this suggests that they are fragments of the longer, 200 m¼. particles, rather than products of self multiplication.