In Escherichia coli, most of the dUMP that is used as a substrate for thymidylate synthetase is generated from dCTP through the sequential action of dCTP deaminase and dUTPase. Some mutations of the dut (dUTPase) gene are lethal even when the cells are grown in the presence of thymidine, but their lethality can be suppressed by extragenic mutations that can be produced by transposon insertion. Six suppressor mutations were tested, and all were found to belong to the same complementation group. The affected gene was cloned, it was mapped by hybridization with a library of recombinant DNA, and its nucleotide sequence was determined. The gene is at 2,149 kb on the physical map. Its product, a 21.2-kDa polypeptide, was overproduced 1,000-fold via an expression vector and identified as dCTP deaminase, the enzyme affected in previously described dcd mutants. Null mutations in dcd probably suppress the lethality of dut mutations by reducing the accumulation of dUTP, which would otherwise lead to the excessive incorporation of uracil into DNA.