The S-layer protein SbsC from Geobacillus stearothermophilus ATCC 12980 is the most prevalent single protein produced by the bacterium and covers the complete bacterial surface in the form of a two-dimensional crystalline monolayer. In order to elucidate the structural features of the assembly domains, several N-terminally truncated fragments of SbsC have been crystallized. Crystals obtained from recombinant fragments showed anisotropic diffraction to a maximum of 3.5 A resolution using synchrotron radiation. The best diffracting crystals were obtained from rSbsC(755-1099), an unintentional in situ proteolytic degradation product of rSbsC(447-1099). Crystals were obtained in two different space groups, P2(1) and P4(1)2(1)2, and diffracted to 2.6 and 3 A resolution, respectively. Native and heavy-atom derivative data have been collected. The structure of the C-terminal part will yield atomic resolution information for the domains that are crucial for the assembly of the two-dimensional lattice.