Homologous replication module genes were identified for four P335 type phages, DNA sequence analysis revealed that all four phages exhibited more than 90% DNA homology for at least two genes,designated rep(2009) and orf17. One of these genes, rep(2009), codes for a putative replisome organizer protein and contains an assumed origin of phage DNA replication (ori(2009)), which was identical for all four phages. DNA fragments representing the ori(2009) sequence confer a phage-encoded resistance (Per) phenotype on lactococcal hosts when they are supplied on a high-copy-number vector. Furthermore, cloning multiple copies of the ori(2009) sequence was found to increase the effectiveness of the Per phenotype conferred. A number of antisense plasmids targeting specific genes of the replication module were constructed. Two separate plasmids targeting rep(2009) and orf17 were found to efficiently inhibit proliferation of all four phages by interfering with intracellular phage DNA replication. These results represent two highly effective strategies for inhibiting bacteriophage proliferation, and they also identify a novel gene, orf17, which appears to be important for phage DNA replication. Furthermore, these results indicate that although the actual mechanisms of DNA replication are very similar, if not identical, for all four phages, expression of the replication genes is significantly different in each ease.