SUMMARYCarnation mottle virus (CarMV) was present in every plant tested of over forty‐four varieties from commercial carnation stocks. Inhibitors of infection in carnation sap prevent transmission to many species, but not toChenopodium amaranticolor, which proved the best indicator and assay host. CarMV infected thirty‐three of 113 plant species inoculated, but onlyDianthusspp. are likely to be affected in practice. Healthy carnations were infected when handled after diseased ones. CarMV was not transmitted byMyzus persicae, through carnation seed, or through dodder (Cuscuta campestris).The type strain occurred in all ‘Sim’ carnations tested; another strain, designated ‘PSR’, occurred in vars. Pink Shibiuya and Orchid Beauty. The two strains were distinguished by the symptoms caused inChenopodium amaranticolorand other plants. The type strain protected against the PSR strain inAtriplex hortensis.An ‘attenuated’ form of the virus, which multiplied much more slowly than the typical form, withstood adverse conditions better.Although the virus concentration decreased in the terminal 1–2 mm. of the carnation shoot apex, CarMV was always detected in the meristem dome. Nevertheless, some virus‐free clones were obtained by aseptic culture of excised tips. CarMV was not eliminated by growing plants at 38°C., although its concentration in the apical tissues was diminished.Sap was infective after dilution to I/200,000, heating (10 min.) to 85°C. (but not 90°C.), storage for up to 81 days at 18°C., or over 3 years at o°C. Purified and concentrated preparations of CarMV were readily obtained from carnation by several methods: they contained ‘spherical’ particles about 28 mμ diameter, were antigenic, and produced a specific light‐scattering virus zone in density‐gradient centrifugation. The S20,wat infinite dilution was 122.Although CarMV shares manyin vitroproperties with eleven other viruses having similar particle shape, it is not serologically related to any of them and differs in other respects.Control of the virus depends on the production and distribution