The pseudoproline residue (ΨPro, L‐2,2‐dimethyl‐1,3‐thiazolidine‐4‐carboxylic acid) has been introduced into heterochiral diproline segments that have been previously shown to facilitate the formation of β‐hairpins, containing central two and three residue turns. NMR studies of the octapeptide Boc‐Leu‐Phe‐Val‐DPro‐ΨPro‐Leu‐Phe‐Val‐OMe (1), Boc‐Leu‐Val‐Val‐DPro‐ΨPro‐Leu‐Val‐Val‐OMe (2), and the nonapeptide sequence Boc‐Leu‐Phe‐Val‐DPro‐ΨPro‐DAla‐Leu‐Phe‐Val‐OMe (3) established well‐registered β‐hairpin structures in chloroform solution, with the almost exclusive population of the trans conformation for the peptide bond preceding the ΨPro residue. The β‐hairpin conformation of 1 is confirmed by single crystal X‐ray diffraction. Truncation of the strand length in Boc‐Val‐DPro‐ΨPro‐Leu‐OMe (4) results in an increase in the population of the cis conformer, with a cis/trans ratio of 3.65. Replacement of ΨPro in 4 by LPro in 5, results in almost exclusive population of the trans form, resulting in an incipient β‐hairpin conformation, stabilized by two intramolecular hydrogen bonds. Further truncation of the sequence gives an appreciable rise in the population of cis conformers in the tripeptide Piv‐DPro‐ΨPro‐Leu‐OMe (6). In the homochiral segment Piv‐Pro‐ΨPro‐Leu‐OMe (7) only the cis form is observed with the NMR evidence strongly supporting a type VIa β‐turn conformation, stabilized by a 4→1 hydrogen bond between the Piv (CO) and Leu (3) NH groups. The crystal structure of the analog peptide 7a (Piv‐Pro‐ΨH,CH3Pro‐Leu‐NHMe) confirms the cis peptide bond geometry for the Pro‐ΨH,CH3Pro peptide bond, resulting in a type VIa β‐turn conformation. © 2009 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 92: 405–416, 2009. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com