We report the first high-resolution structural data for the 14/15-helix, a secondary structure that is formed by oligomers with a 1: 1 alternation of alpha- and beta-amino acid residues. Previously, we concluded from NMR data that short alpha/beta-peptides containing cyclopentane-constrained beta-residues display rapid interconversion between two helical folding patterns, the 11-helix (i ,i +3 C=O center dot center dot center dot H-N H-bonds) and the 14/15-helix (i ,i +4 C=O center dot center dot center dot H-N H-bonds). Subsequent work in other laboratories, however, has called this hypothesis into question. Partial support for our original hypothesis was obtained when we obtained the first crystal structure in this alpha/beta-peptide series, which revealed an 11-helical conformation. The present report of a 14/15-helical conformation strengthens the original hypothesis.