The brome mosaic virus (BMV) RNA-dependent RNA polymerase (RdRp) directs template-specific synthesis of (−)-strand genomic and (+)-strand subgenomic RNAs in vitro. Although the requirements for (−)-strand RNA synthesis have been characterized previously, the mechanism of subgenomic RNA synthesis has not. Mutational analysis of the subgenomic promoter revealed that the +1 cytidylate and the +2 adenylate are important for RNA synthesis. Unlike (−)-strand RNA synthesis, which required only a high GTP concentration, subgenomic RNA synthesis required high concentrations of both GTP and UTP. Phylogenetic analysis of the sequences surrounding the initiation sites for subgenomic and genomic (+)-strand RNA synthesis in representative members of the alphavirus-like superfamily revealed that the +1 and +2 positions are highly conserved as a pyrimidine–adenylate. GDP and dinucleotide primers were able to more efficiently stimulate (−)-strand synthesis than subgenomic synthesis under conditions of limiting GTP. Oligonucleotide products of 6-, 7-, and 9-nt were synthesized and released by RdRp in 3–20-fold molar excess to full-length subgenomic RNA. Termination of RNA synthesis by RdRp was not induced by template sequence alone. Our characterization of the stepwise mechanism of subgenomic and (−)-strand RNA synthesis by RdRp permits comparisons to the mechanism of DNA-dependent RNA synthesis.