The B. stearothermophilus cdd gene encoding cytidine deaminase (cytidine/2’deoxycytidine aminohydrolase; EC 3.5.4.5) was isolated through shotgun cloning by complementation of an E. coli cdd mutation. From the primary cloned plasmid, pJSC101, 1.34Kbp of the EcoRI₁/PstI₂ fragment containing the cdd gene in pJSC201 was introduced to the sequencing. The complete sequence of 1340 base pairs of an EcoRI₁ /PstI₂ fragment revealed to one single polypeptide chain. The open reading frame of the cdd gene is located from the nucleotide 457 to 1284 with 274 amino acids. The calculated molecular mass deduced from the sequence was about 31,100. This value is corresponded with the molecular weight from the plasmid-coded polypeptide 33,000 in minicell experiment. Considering the molecular mass of native form of the enzyme which determined as 58,000 in Sephadex G-100 column chromatography, the cytidine deaminase may be consist of two identical subunits, each has a molecular mass of 31,100. Analysis of the nucleotide sequence of the cdd gene will be presented.