The consequences on replication of collisions between phi29 DNA polymerase, a monomeric replicase endowed with strand displacement capacity, and the transcription machinery have been studied in vitro. Codirectional collisions with stalled transcription ternary complexes at four different promoters in the phi29 genome were found to block replication fork progression. Upon collision, the DNA polymerase remained on the template and was able to resume elongation once the RNA polymerase was allowed to move. Collisions with RNA polymerase molecules moving in the same direction also interfered with replication, causing a decrease in the replication rate. These results lead to the proposal that in bacteriophage phi29 a transcription complex physically blocks the progression of a replication fork. We suggest that temporal regulation of transcription and the low probability that the replication and transcription processes colocalize in vivo contribute to achieving minimal interference between the two events.