Mutation of the capR(lon), capS, or capT genes in Escherichia coli K-12 causes overproduction of capsular polysaccharide leading to a mucoid phenotype. Several of the enzymes involved in capsular polysaccharide synthesis are derepressed in cap mutants. Previously it was shown that uridine diphosphate-glucose (UDPG) pyrophosphorylase, an enzyme involved in the synthesis of three of the nucleotide sugar precursors of the capsule, is derepressed in capR mutants. The control of galU, the gene which codes for UDPG pyrophosphorylase, is described in this study. In addition, it has been found that the enzyme is also derepressed in capS and capT mutants. The effect of galU gene dosage in cap mutants and the wild-type strain (all lysogenic for phi80) was studied by infecting them with the purified transducing phage phi80dgalU. The level of UDPG pyrophosphorylase increased in proportion to the number of galU copies added. The rate of enzyme synthesis in the mutants was about sixfold higher than in the wild type per galU gene added for multiplicities of infection from one to twenty. Thus, all the galU copies added to the wild-type lysogen were repressed. We obtain greater than 20 galU copies per cell by infecting the nonlysogenic strain which allows multiplication of phi80dgalU. With some number of galU copies greater than 20, the rate of UDPG pyrophosphorylase synthesis in the wild type approaches the mutant rate of synthesis. The results suggest that there may indeed be a galU repressor pool in the cell which can be completely titrated. This pool must be composed of more than 20 galU repressor molecules. Since the capR, capS, and capT gene products or combinations thereof are known to control other widely separated operons of the cell besides the galU gene, it is postulated that the galU repressor may be capable of binding other operators. This would account for the relatively large pool of galU repressors per cell.