An account is given of a composite virus disease of tobacco for which the name tobacco rosette has been suggested.The two component viruses, named the mottle and vein-distorting viruses respectively, have been separated, and their symptomatology and methods of transmission described. The mottle virus is both sap and aphis-transmitted, but the vein-distorting virus is aphis-borne only.The symptoms and histopathology of the complex disease in the tobacco plant are dealt with in some detail. There are three main types of symptoms: (1) intense rosetting, (2) splitting of the tissues, (3) formation of enations on the under-surface of the leaves.The splitting of the tissues has been examined microscopically, and a number of photomicrographs are given illustrating the formation of the fissures. It is suggested that there is a concentration of virus in the cambium which prevents the formation of the normal xylem. Abnormal tissue and giant cells are formed in the cortex and pith. This appears to set up stresses which cause the splitting.The insect vector of the complex disease is the aphis Myzus persicae Sulz. Another aphis, M. pseudosolani Theob., is also a vector but is less efficient than M. persicae.The writer's thanks are due to Prof. F. T. Brooks, F.R.S., with whom he discussed the histopathology of the rosette disease, to Dr Roy Markham for taking the photographs illustrating Pl. I, figs. 2–5, and to Mr Charles Harpley of the Molteno Institute for his assistance in taking' the photomicrographs.