The aphA gene of Salmonella enterica sv. Typhimurium strain MD6001 was cloned in the multicopy plasmid pBluescript SK^- .The recombinant AphA protein was purified to homogeneity. The protein crystallized in the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a= 112.4,b= 130.2,c= 139.6 Â. Consistent with the self-rotation function, there are two tetramers in the asymmetric unit,indicating a solvent content of _˜ 54%. The crystals are composed of biologically active AphA molecules.