To decrease the costs of producing the anti-HIV drug, lamivudine, an enzymatic conversion process was developed instead of the traditional chemical method. Thermostable cytidine deaminase was over-produced by cloning the cdd gene into E. coli JF611/pCJH53 from Bacillus caldolyticus. The purified cytidine deaminase was recovered from the lysate of the recombinant E. coli JF611/pCJH53 by removing heat-denatured proteins and eluting sequential chromatography. When the enzyme was used to deaminate (-)-beta -l-(2R,5S)- and (+)-beta -d-(2S,5R)-1,3-oxathiolanyl-cytosine, about 68% of the (+)-beta -d-(2S,5R)-1,3-oxathiolanyl-cytosine was deaminated into the corresponding (+)-thiauridine maximally.