Abstract Residues 24 to 35 of T4 lysozyme correspond to the second and third strands of a region of β-sheet that is highly conserved in all known lysozyme and chitinase structures. To evaluate the intrinsic propensity of these amino acid residues to form a defined structure they were added at the C terminus of the native protein, together with a dipeptide linker. Two crystal structures of this active, mutant protein were obtained, to 1.9 Å and 2.3 Å resolution, respectively. Even though the crystal conditions are similar, the appended sequence adopts very different secondary structures. In one case it is weakly structured and appears to extend through the active-site cleft, perhaps in part adding an extra strand to the original β-sheet. In the other crystal form the extension is largely α-helical. The formation of these alternative structures shows that the sequence does not have a strong intrinsic propensity to form a unique fold (either β-sheet or otherwise). The results also suggest that structural conservation during evolution does not necessarily depend on sequence conservation or the conservation of folding propensity.