phi Lf is a filamentous phage of Xanthomonas campestris pv. campestris. In this study, the origin for phi Lf replication was located in a 121-bp TaqI fragment within gII, the gene encoding the replication initiation protein. This fragment, ligating with a GmR catridge, was able to be maintained as a plasmid (pT2) in strain Xc17 with the gII being provided in trans. ssDNA of pT2 was detected in the cells, indicating that pT2 may replicate by a rolling circle replication mechanism. Upon superinfection of Xc17 containing pT2 with phi Lf, transducing particles containing ssDNA of pT2 were released, suggesting the presence of packaging signal in the 121-bp TaqI fragment. This fragment contains a sequence homologous to the nicking sites for superfamily I Rep proteins of the rolling circle-replicating replicons, in concert with the presence of conserved amino acid sequence motifs of the superfamily I Rep proteins in the phi Lf gIIP.