The structures of the protected tetrapeptides Z-Aib-Aib-Aib-Val-OtBu (I) and Z-Val-Aib-Aib-Gln-OtBu (II), which contain the conformationally constrained residue α-aminoisobutyric acid (Aib), have been studied by X-ray crystallography. Both molecules in the asymmetric unit of I adopt left-handed 310-helical conformation, stabilized both by two 4 → 1 intramolecular hydrogen bonds. The structure of II consists of a β-turn of type II and a consecutive β-turn of type III′ and is extended at the C-terminus. This structure is stabilized by three intramolecular hydrogen bonds. All non-Aib residues (Val, Gln) in I and II adopt rather unusual backbone conformations compared with other helical structures in proteins.