Endoglucanase CelC from Clostridium thermocellum expressed in Escherichia coli has been crystallized in two different crystal forms by the hanging drop method. Crystals of form I were grown with polyethylene glycol as a precipitant. They are orthorhombic, space group P2(1)2(1)2(1), with cell dimensions a = 51.4 A, b = 84.3 A, and c = 87.5 A. Crystals of form II, obtained in ammonium sulfate solutions, belong to the tetragonal space group P4(1)2(1)2 (or P4(3)2(1)2) with cell dimensions of a = b = 130.7 A and c = 69.6 A. Diffraction data to 2.8 A resolution were observed for both crystal forms with a rotating anode generator. Preliminary oscillation images of the orthorhombic form I crystals using a synchrotron radiation source show diffraction to 2.2 A resolution, indicating that these crystals are suitable for high resolution crystallographic analysis.