Technical Abstract: The blue chloroform-soluble bacterial metabolite pyocyanin (1-hydroxy-5-methyl-phenazine) contributes to the survival and virulence of Pseudomonas aeruginosa, an important Gram-negative opportunistic pathogen of humans and animals. Little is known about the two enzymes, designated PhzM and PhzS, that function in the synthesis of pyocyanin from phenazine-1-carboxylic acid. In this study, the FAD-dependent monooxygenase PhzS was purified and crystallized from lithium sulfate/ammonium sulfate/sodium citrate pH 5.5. Native crystals belong to space group C2, with unit-cell parameters a = 144.2, b= 96.2, c= 71.7 angstrom, alpha = gamma = 90, beta = 110.5 degrees. They contain two monomers of PhzS in the asymmetric unit and diffract to a resolution of 2.4 angstrom. Seleno-L-methionine-labelled PhzS also crystallizes in space group C2, but the unit-cell parameters change to a = 70.6, b = 76.2, c = 80.2 angstrom, alpha = gamma = 90, beta = 110.5 degrees and the diffraction limit is 2.7 angstrom.