RNA polymerase activity capable of synthesizing long-chain, heteropolymeric RNA is associated with specific ribonucleoprotein complexes present in the cytoplasm of cells infected with vesicular stomatitis virus (VSV). The synthesis is independent of added exogenous template and the product labelled with GTP precursors can be totally annealed with excess virion RNA. The enzyme activity is therefore a transcriptase similar to that observed in association with the mature VSV virion.