Oligomers that contain both alpha- and beta-amino acid residues in a 1:1 alternating pattern have recently been shown by several groups to adopt helical secondary structures in solution. The beta-residue substitution pattern has a profound effect on the type of helix formed and the stability of the helical conformation. On the basis of two-dimensional NMR data, we have previously proposed that beta-residues with a five-membered ring constraint promote two different types of alpha/beta-peptide helix. The "11-helix" contains i,i+3 C=O center dot center dot center dot H-N hydrogen bonds between backbone amide groups; these hydrogen bonds occur in 11-atom rings. The alpha/beta-peptide "14/15-helix" contains i,i+4 C=O center dot center dot center dot H-N hydrogen bonds, which occur in alternating 14- and 15-atom rings. Here we provide crystallographic data for 14 alpha/beta-peptides that form the 11-helix and/or the 14/15-helix. These results were obtained for a series of oligomers containing P-residues derived from (S,S)-trans-2-aminocyclopentanecarboxylic acid (ACPC) and alpha-residues derived from (x-aminoisobutyric acid (Aib) or L-alanine (Ala). The crystallized alpha/beta-peptides range in length from 4 to 10 residues. Nine of the alpha/beta-peptides display the 11-helix in the solid state, three display the 14/15-helix, and two display conformations that contain both i,i+3 and i,i+4 C=O center dot center dot center dot H-N hydrogen bonds, but not bifurcated hydrogen bonds. Only 3 of the 14 crystal structures presented here have been previously described. These results suggest that longer alpha/beta-peptides prefer the 14/15-helix over the 11-helix, a conclusion that is consistent with previously reported NMR data obtained in solution.