A defective interfering particle derived from the heat-resistant strain of vesicular stomatitis virus was analyzed for the presence of virion-associated RNA polymerase (nucleosidetriphosphate:RNA nucleotidyltransferase, EC 2.7.7.6) activtiy. The RNA synthesizing capacity of the defective particles in vitro was similar to that of the wild-type virus. Characterization of the RNA produced in vitro indicated that the defective particles were able to synthesize vesicular stomatitis virus leader RNA and four virus mRNA species that sediment at 12-18 S. These RNA products were identical to the mRNAs synthesized in vitro by the wild-type virus in regard to size, polyadenylation, capping, and methylation. In contrast to the wild-type virus, the purified defective particles did not synthesize the large mRNA species sedimenting at 31 S in vitro. Possible mechanisms of homotypic and heterotypic interferences shown by this defective particle are discussed.