Infrared Spectra in the 700—200 cm−1 and in the amide I and II regions were measured for poly(L-Ala-L-Ala-D-Ala), poly(L-Ala-L-Ala-D-Ala-D-Ala), poly(L-Ala-L-Ala-L-Ala-D-Ala), and poly(L-Ala-D-Ala-L-Ala-D-Ala) with different degrees of polymerization. The spectra showed that the films cast from aqueous solutions of poly(L-Ala-L-Ala-D-Ala), poly(L-Ala-L-Ala-D-Ala-D-Ala), and poly(L-Ala-L-Ala-L-Ala-D-Ala) take disordered conformations containing appreciable amounts of L-alanine residues with local right-handed α-helix-like structures and the D-alanine residues with local left-handed α-helix-like structures. The disordered conformations were converted to the extended β-form when treated with ethanol. The latter conformation is also found in water-insoluble portions of poly(L-Ala-L-Ala-D-Ala-D-Ala) and poly(L-Ala-L-Ala-L-Ala-D-Ala). On the other hand, poly(L-Ala-D-Ala-L-Ala-D-Ala) has a conformation different from those taken on other sequential polypeptides studied in this paper. The above findings were confirmed also by the measurement of the Raman spectra of poly(L-Ala-L-Ala-D-Ala) and poly(L-Ala-D-Ala-L-Ala-D-Ala).