The structure of the synthetic, protected peptide Z-(Aib)6-OBut (C36H58O9N6, Mw = 718.89) which contains the conformationally constrained residue α-aminoisobutyric acid (Aib) has been determined by X-ray crystallography. The title compound crystallizes in P21/c with a = 8.992(4) Å, b = 19.382(4) Å, c = 27.451(10) Å, β = 115.63(4)°, V = 4360.5(8) Å3, Z = 4, Dx = 1.14(2) gcm−3. The final R after refinement is 0.088 for 5221 observed reflections. The hexapeptide adopts a 310 helical conformation with a reversal of the sense of the helix at the C-terminal Aib residue. The helix consists of three β-turns of type III and one β-turn of type I. Both senses of the helix occur in the crystal. The sequence – (Aib)6 – possesses a remarkable conformational flexibility. In the crystal the molecules are hydrogen-bonded head-to-tail, forming infinitely long, parallel, helical columns along .