Azoreductases from Bacillus sp. B29 are NADH-dependent flavoenzymes which contain a flavin mononucleotide (FMN) as a prosthetic group and exist as homodimers composed of 23 kDa subunits. These enzymes catalyze the reductive degradation of various azo compounds by a ping-pong mechanism. In order to determine the structure-function relationship of the azo-dye reduction mechanism, an X-ray crystallographic study of azoreductases was performed. Selenomethionine-labelled AzrA (SeMet-AzrA) and AzrC were crystallized by the hanging-drop vapour-diffusion method. A crystal of SeMet-AzrA diffracted to 2.0 A resolution and was determined to belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 56.9, b = 69.0, c = 105.4 A. The native crystals of AzrC belonged to space group C2, with unit-cell parameters a = 192.0, b = 56.6, c = 105.5 A, beta = 115.7 degrees , and diffracted to 2.21 A resolution.