The repetitive region of the circadian clock gene period in Drosophila pseudoobscura consists predominantly of a pentapeptide sequence whose consensus is NSGAD. In D. melanogaster, this region is replaced by a dipeptide Thr–Gly repeat, which plays a role in the thermal stability of the circadian phenotype. The Thr–Gly repeat has been shown to form a type II or III β–turn, whose conformational monomer is (Thr–Gly)3. Here we report, using conformational analyses, that both an NSGAD pentapeptide, and a polymer of the same sequence, form type II β–turns. Thus two peptide sequences, whose amino acid composition is very different, nevertheless form the same secondary structure. The implications of these structures for clock function are discussed.