A new satellite-like replicating RNA found in association with cucumber mosaic virus type 1 (CMV-1), one of the earliest CMV isolates described (Doolittle, 1920), was identified and partially characterized. CMV-1-associated RNA 5, or (1)CARNA 5, was isolated from CMV-1 propagated in tomato, where it failed to induce lethal necrosis. In this way it was distinguished from CARNA 5 with the tomato necrosis-causing ability, which was redesignated (n)CARNA 5. Although (1)CARNA 5 and (n)CARNA 5 could be separated by 7.5% polyacrylamide gel electrophoresis under certain conditions, the molecules are probably identical in size, or nearly so. (1)CARNA 5 and (n)CARNA 5 have only limited nucleotide sequence homology. This is also evident from the different multitransitional absorbance-temperature profiles that can be obtained during the thermal denaturation of their respective double-stranded forms. In mixed infections in tomato, the replication of (1)CARNA 5 was also supported by CMV strains WT or S, while (n)CARNA 5 was supported by CMV-1. The biologically different (1)CARNA 5 and (n)CARNA 5 can be considered sequence variants among a group of small satellite-like RNA molecules dependent upon CMV for their replication.