In the sixth edition of Plant Viruses, each section has been brought up to date and some additions made. A short account is given of a new technique, the protoplast-culture of plant viruses. The omission in the fifth edition of the Mycoplasma-like organisms has been criticized on the grounds of the close simi­ larity of symptom expression and techniques of study between Mycoplasmas and plant viruses. To meet this criticism Chapter 16, which gives a brief account of these organisms, is included. Some of the plates have been changed and new ones added. Grateful acknowledgment is due to Dr Aaron KIug F.R.S. and his colleagues, to Drs D. A. Govier, Basil Kassanis F.R.S. and Karl Maramorsch for permission to use their recent work. Acknowledgement is also due to several friends who have supplied prints of illustrations from their published work; credit has been given to authors in the illustration legends. Mr Denis C. Ingram, editor to Chapman and Hall, has been most helpful and co-operative throughout.