Potato mop-top virus particles, purified from systemically infectedNicotiana benthamiana plants and then disrupted by heating with sodium dodecyl sulphate and 2-mercaptoethanol, contained only a single polypeptide of Mr 19 100 detectable by polyacrylamide gel electrophoresis. Single-stranded RNA preparations from virus particles, when subjected to electrophoresis in an agarose gel containing methylmercuric hydroxide as a denaturant, were shown to contain approximately equal proportions of three RNAs of 6.5, 3.2 and 2.5 kb. Double-stranded RNA preparations extracted from systemically infectedN. benthamiana leaves or from locally infectedN. debneyi leaves, was shown by polyacrylamide gel electrophoresis to contain a major species of 3.2 kbp and two minor species of 6.5 and 2.4 kbp.