A plasmid carrying the arginine repressor gene (argR) of Escherichia coli was obtained out of the Clarke and Carbon colony bank which contains clones of individual ColE1 plasmids carrying fragments of the E. coli chromosome. A shortened derivative of this plasmid was isolated and analyzed by restriction mapping and by cloning fragments of it into plasmid pBR322. The argR gene was located on a 2 kb fragment.