Defective bacteriophage PBSX, a resident of all Bacillus subtilis 168 chromosomes, packages fragments of DNA from all portions of the host chromosome when induced by mitomycin C. In this study, the physical process for DNA packaging of both chromosomal and plasmid DNAs was examined. Discrete 13-kilobase (kb) lengths of DNA were packaged by wild-type phage, and the process was DNase I resistant and probably occurred by a head-filling mechanism. Genetically engineered isogenic host strains having a chloramphenicol resistance determinant integrated as a genetic flag at two different regions of the chromosome were used to monitor the packaging of specific chromosomal regions. No dramatic selectivity for these regions could be documented. If the wild-type strain 168 contains autonomously replicating plasmids, especially pC194, the mitomycin C induces an increase in size of resident plasmid DNA, which is then packaged as 13-kb pieces into phage heads. In strain RB1144, which lacks substantial portions of the PBSX resident phage region, mitomycin C treatment did not affect the structure of resident plasmids. Induction of PBSX started rolling circle replication on plasmids, which then became packaged as 13-kb fragments. This alteration or cannibalization of plasmid replication resulting from mitomycin C treatment requires for its function some DNA within the prophage deletion of strain RB1144.