All methyl-beta-D-galacturonide-positive mutants isolated from Escherichia coli K-12 carry constitutive mutations for beta-glucuronidase (UID) synthesis. Most of these mutants are specific for UID synthesis and are distributed in three classes according to the derepression level of UID. Each specific mutant carries a mutation(s) near uidA, the structural gene for UID, at min 30.5 of the E. coli K-12 linkage map. The expression of UID synthesis in F-merodiploid strains carrying these mutations permits discrimination between dominant and recessive constitutivity over the wild-type allele. The first kind of mutation (dominant) should affect the operator site uidO of the structural gene uidA; the second type of mutation (recessive) should affect a regulatory gene, uidR, operating through a negative control. The isolation of mutants bearing at this locus superrepressed mutations, which can revert to produce a constitutive phenotype, confirms the occurrence of such a regulatory gene. The partially derepressed uidR mutants of the first class are normally inducible and remain constitutive at low temperature; their UID has the same thermal sensitivity as in the wild-type strains. The occurrence of similar regulatory gene mutants has been recently described in the lactose system (Shineberg, 1974).