Cucumber mosaic virus (CMV) is an isometric virus which infects a wide range of hosts in the temperate regions of the world (Kaper & Waterworth, 1981). CMV genomic R N A s consist of four components (mol. wt. 1.09, 0-98, 0.71 and 0.33, all x 106). The three largest components carry all the information required for virus replication, whereas the fourth component corresponds to the part of the genome in R N A 3 that carries the message for the coat protein (Kaper & Waterworth, 1981). Particles of some strains also contain a satellite R N A (mol. wt. 1-08 x 105). The instability of CMV particles in solutions of SDS suggested that they are stabilized by strong p r o t e i n R N A interactions (Boatman & Kaper , 1976) and this was confirmed by Piazzolla et al. (1977a). Structural studies by small angle neutron diffraction demonstra ted that CMV R N A penetrates to some extent into the protein shell (Jacrot et al., 1977) and the results of hypochromicity experiments suggested that the R N A inside the virus particle has a helical content of about 7 0 ~ (Kaper et al., 1965). Circular dichroism (CD) spectroscopy has been used to investigate plant virus structure and stability (Isenberg et al., 1971 : Incardona et al., 1973; Piazzolla et al., 1977b; Tamburro et al., 1978; Denloye et al., 1978; Odumosu et al., 1981). CMV particles seemed to be part icularly interesting because of the presence of the satellite RN A and the recent proposal that it interacts with CMV by a mechanism that inhibits viral R N A replication (Kaper, 1984). In this paper we describe observations on the conformation of CMV particles and of CMV R N A components under different experimental conditions.