A fine structure map of the phoR region of E. coli , mutations of which affect the rate of alkaline phosphatase synthesis, was constructed by Hfr x F- crosses. Mutations causing three different phenotypes (previously reported as phoRa, phoRb, phoRc (Garen and Echols 1962a, b)) are clustered in three closely linked genetic loci. PhoR mutants of all three types, including the phoRb type not previously tested, are recessive to wild-type phoR+. In addition, phoRa and phoRc complement each other, while phoRa and phoRb do not. Our results support the hypothesis of Morris et al. (1974) that phoRc mutants represent a cistron (phoB) different from phoR.