Abstract The low-molecular-weight fifth RNA component encapsidated in cucumber mosaic virus (CMV) has recently been identified as a satellite-like or defective RNA, dependent on a helper virus for its replication. Of six CMV strains studied, five were found to have this RNA 5 associated in major proportions after one or more serial transfers in tobacco. After passage(s) in squash, the viruses (with the exception of CMV strain R) contained only small amounts of RNA 5. In tobacco, increased RNA 5 proportions in the CMV preparations isolated were correlated with a decrease in symptoms of the plants, a decrease in specific infectivities, yields, and in the proportions of some of the genomic RNAs (particularly RNA 1) in the virus, indicating a general suppression of helper virus synthesis. The RNA 5 components of the five strains comigrated in polyacrylamide electrophoresis on 4% gels and thus seem to have very similar or identical molecular weights. In tobacco, addition of purified RNA 5 from different strains to inocula consisting of RNA 1 + 2 + 3 from CMV-S induced replication of RNA 5. Similarly, mixtures of RNA 5 and RNA 1 + 2 + 3 from CMV-S and CMV-R, and vice versa, were found to produce RNA 5 in the progeny. This made it possible to establish that, for these two strains, the amount of RNA 5 produced in squash was determined by the helper and not by the RNA 5 itself. Thus, with CMV the amount of RNA 5 associated with virions seems to be a characteristic determined by the interaction of the host plant with the helper virus and not with the RNA 5. This interaction could also be an important factor in the genesis of helper-dependent replicating RNAs. A code name CARNA 5 was introduced for CMV-associated RNA 5.