The bacteriophage fl genome contains an intergenic region (IG) 508 nucleotides long that does not code for any known protein. By using chimeric plasmids harboring different portions of this region as well as neighboring parts of the fl genome, we have identified three functional domains: (1) the proximal part of gene II, responsible for the prevention of fl and f2 adsorption and penetration (IP2 phenotype); (2) a part in the middle of the IG responsible for the fl "functional origin of replication;" and (3) a sequence at the gene IV/IG junction important for particle morphogenesis. We have shown that these regions cab be inserted into separate sites of a plasmid and function independently of one another.