Avian reovirus fibre, a homotrimer of the sigmaC protein, is responsible for primary host-cell attachment. Using the protease trypsin, a C-terminal sigmaC fragment containing amino acids 156-326 has been generated which was subsequently purified and crystallized. Two different crystal forms were obtained, one grown in the absence of divalent cations and belonging to space group P6(3)22 (unit-cell parameters a = 75.6, c = 243.1 A) and one grown in the presence of either zinc or cadmium sulfate and belonging to space group P321 (unit-cell parameters a = 74.7, c = 74.5 A and a = 73.1, c = 69.9 A for the Zn(II)- and Cd(II)-grown crystals, respectively). The first crystal form diffracted synchrotron radiation to 3.0 A resolution and the second form to 2.2-2.3 A. Its closest related structure, the C-terminal fragment of mammalian reovirus fibre, has only 18% sequence identity and molecular-replacement attempts were unsuccessful. Therefore, a search is under way for suitable heavy-atom derivatives and attempts are being made to grow protein crystals containing selenomethionine instead of methionine.