The crystal structure determination of the heptapeptide Boc-Val-Ala-Leu-Aib-Val-Ala-Phe-OMe reveals two peptide helices in the asymmetric unit. Crystal parameters are: space group P2(1), a = 10.356(2) A, b = 19.488(5) A, c = 23.756(6) A, beta = 102.25(2) degrees, V = 4685.4 A3, Z = 4 and R = 5.7% for 7615 reflections [I > 3 sigma (I)]. Both molecules adopt largely alpha-helical conformations with variations at the C-terminus. Helix type is determined by analysing both 4-->1 and 5-->1 hydrogen-bond interactions and comparison with the results of analysis of protein structures. The presence of two 4-->1 hydrogen-bond interactions, besides four 5-->1 interactions in both the conformations provides an opportunity to characterize bifurcated hydrogen bonds at high resolution. Comparison of the two helical conformations with related peptide structures suggests that distortions at the C-terminus are more facile than at the N-terminus.