The final step in lysine biosynthesis in bacteria, the conversion of meso-diaminopimelate to L-lysine, is catalyzed by the only known D-amino-acid decarboxylase, diaminopimelate decarboxylase (DDC). The Escherichia coli DDC has been cloned, overexpressed in E. coli with a carboxy-terminal polyhistidine purification tag and crystallized from lithium sulfate. The protein is intensely yellow, owing to the pyridoxal-5'-phosphate cofactor, and is enzymatically active. Large well ordered crystals, belonging to space group P6(1)22 with unit-cell parameters a = b = 98.6, c = 177 A, make high-resolution X-ray diffraction studies possible to characterize the residues important in stereospecific decarboxylation and reprotonation during catalytic turnover.