N4 phage, which infects E. coli K-12 strains, could not infect E. coli K-12 strains containing rtn(resistant to N4) gene on plasmids, which was isolated from Proteus vulgaris ATCC 13315. The region of rtn gene for Rtn phenotype was reduced to the 1.7 kb HincII-AccI fragment, and rtn gene seemed to have its own promoter. This putative promoter was present in 107 bp HindII-DraI fragment, and known to be functional in E. cole K-12, which is supported by the fact that phenotype of a subclone, pRMG103A1B which does not contain the 107 bp fragment, was dependent on the existance of a functional promoter in the upstream of rtn gene, and that the 107 bp fragment had promoter activity when located in the upstream of structural gene of galactodinase of E. coli. The promoter-bearing fragment contains two overlapping putative promoter sequences, both of which show a fit in eight of twelve nucleotides with consensus sequences of E. coli promoters at the -35 and -10 regions.