Summary RNAs prepared from particles of ten plant viruses by a new procedure in which detergent and sodium perchlorate were used were compared with RNAs obtained by the standard two-phase phenol technique. On the basis of u.v. extinction spectra, yield, component composition and infectivity of the RNAs, the NaClO4 method was judged to be as good as and often considerably better than, the phenol method. Moreover, the NaClO4 method is simpler and faster, and it gives good yields of RNA from viruses from which it is usually difficult to isolate RNA by the phenol method.