A gene from Proteus bulgaris ATCC13315, which has been cloned in pUC9 vector and propagated in E.coli HB101, conferred phage resistance. This phage resistance was not due to restriction modification system, but a novel system, as judged both by in vivo tests and by the activity of the cell extracts in vitro. From this clone(pRMG101), a subclone(pRMG101A8) has been constructed that contains the phage resistance system on a 2.4 kilobase DNA fragment. Digestion of the pRMG101A8 plasmid with HindIII endonuclease has yielded two types of subsubclones which have 1.4 kb, or 0.7 kb fragment, resulting in disappearance of phage resistance. Another sub-subclone with 1.2 kb fragment also contains no phage resistance phenotype. Sequence of a DNA segment containing the phage resistance gene was determined by the chain termination method of Sanger using M13 clones. Codon assignment for possible structural gene, presence of representative promoter region and subcloning experiments idicate that the coding region include the two open translational frames of sufficient length to accomodate the phage resistance system.