Fibritin, a 52-kDa product of genewacof bacteriophage T4, forms fibrous “whiskers” that connect to the phage tail and facilitate the later stages of phage assembly. Preliminary experiments suggest that fibritin is a trimer, and its predominant central part has a parallel α-helical coiled-coil structure. To investigate the oligomerization and function of fibritin, we have designed and studied two related deletion mutants, denoted M and E, that consist of its last 75 and 120 amino acids, respectively. Both proteins contain part of the coiled-coil region and the 29 amino acid carboxy-terminal domain essential for the trimerization of fibritin. The proteins are expressed as a soluble product in anEscherichia colisystem. We have obtained crystals of fibritins M and E. Complete native X-ray diffraction data sets have been collected to 1.85 and 2.7 Å resolution, respectively. The crystals have space groupP3 witha= 44.3 Å,c= 91.3 Å (fibritin M) andR32 witha= 41.2 Å,b= 358.7 Å (fibritin E) in the hexagonal setting. Symmetry and packing considerations show that fibritin is a triple coiled coil.