A 24 kDa N-terminal fragment of the Escherichia coli DNA gyrase B protein has been crystallized in the presence of novobiocin. One crystal form has been obtained that is orthorhombic, P2(1)2(1)2(1), with unit cell dimensions a = 40.3 A, b = 47.7 A, c = 111.9 A. The asymmetric unit of this crystal form contains one molecule (Vm = 2.24 A3/Da). Complete native data have been collected to 2.5 A resolution. This same protein fragment has also been crystallized in the presence of GR122222X, an inhibitor that is structurally related to cyclothialidine. These crystals also exhibit P2(1)2(1)2(1) symmetry but have unit cell dimensions of a = 68.8 A, b = 68.6 A, c = 48.6 A. The Vm value of this crystal form is 2.39 A3/Da, assuming one molecule in the asymmetric unit, and native data have been collected to 2.0 A resolution. Molecular replacement studies of both complexes are underway.