The sedimentation analysis and polyacrylamide gel electrophoresis of RNA extracted from Lumbo and Tahyna viruses, which belong to the California group of the Bunyamwera supergroup of arboviruses, demonstrated the existence of three RNA species with molecular weights of 2.9 × 106, 2 × 106 and 0.5 × 106 daltons. In control experiments, the same extraction procedure failed to break down the 42S Sindbis virus RNA. The three RNA species cannot be denatured by heating or DMSO treatment.