Alginate lyase (EC 4.2.2.3) from a soil bacterium (Flavobacterium sp.), which was overexpressed in Bacillus subtilis, has been crystallized by the vapor-diffusion method. Tetragonal crystals were obtained using ammonium sulfate as a precipitant. The crystal was resistant to X-ray radiation damage and diffracted to about 2.2 Angstrom resolution. The diffraction pattern indicated that the crystal belongs to the orthorhombic system, space group P2(1)2(1)2(1) with unit-cell dimensions of a=82.7, b=142.1 and c=80.9 Angstrom. It is supposed that the asymmetric unit consists of two alginate lyase molecules.