The replication of Sindbis virus was studied in primary chick embryo fibroblasts treated with actinomycin D. The cellular site of viral RNA and coat protein synthesis was found to be localized in the cytoplasmic reticulum. Two different species of RNA were identified in association with the reticulum, namely a double-stranded RNA with a sedimentation constant of 20 S and RNA with a sedimentation constant of 25 S. The single-stranded RNA molecules extracted from the 130 S virus particles had a sedimentation coefficient of 40 S. The time course of the association of viral RNA and coat proteins demonstrated that both components appear simultaneously in the 130 S viral ribonucleoprotein particles within 15 min.