The interaction of DNA replication forks with both stationary and transcribing RNA polymerase molecules has been examined in vitro, usfng the multlenzyme 14 bacteriophage DNA repllcatlon system and purified E. coli RNA polymerase. We have found that a single statkxtary RNA polymerase molecule can block the movement of the 14 repliition fork when bound to a promoter on a double-stranded fd DNA template. When tranacrfption is allowed (in the same direction as replication), the replication fork appears to follow the moving RNA polymetase molecule at the relatively slow rate of transcription. The barriers to fork movement formed by E. coli RNA polymerase are eliminated by the addiin of small amounts of a purified T4-encoded DNA helicase, the product of the dda gene. We find that replication complexes containing the dda protein cause stationary RNA polymerase molecules to dissociate from the DNA.