Cyclodextrin glucanotransferase from Bacillus stearothermophilus TC-91 has been crystallized from an ammonium sulfate solution by the dialysis equilibrium method. The crystals belong to the orthorhombic system, space group P2(1)2(1)2(1), with cell dimensions of a = 125.5 A, b = 88.1 A, and c = 81.5 A. The crystals appear to be suitable for X-ray structure analysis, diffracting to at least 2.1 A and being resistant to radiation damage.