DNA oligonucleotides with appropriate sequences can form a stable duplex in which the two strands are paired in a parallel orientation instead of as the conventional antiparallel double helix of B-DNA. In parallel-stranded DNA (ps-DNA) base pairing is noncanonical with the glycosidic bonds in a trans orientation. The two grooves are equivalent. We have synthesized DNA duplexes consisting of a central parallel-stranded (dA)15.(dT)15 tract flanked by normal antiparallel regions, and ligated them into the pUC18 plasmid. The effect of negative supercoiling on the covalently closed circular molecules was studied by two-dimensional agarose gel electrophoresis and by chemical modification with OsO4-pyridine (Os,py) and diethylpyrocarbonate (DEPC). The following results were obtained: (i) The ps insert, and by inference ps-DNA in general, adopts a right handed helical form. (ii) Upon increasing the negative superhelix density (-sigma) to greater than 0.03 the 15 bp ps insert undergoes a major transition leading to a relaxation corresponding to a reduction in twist of approximately 2.5 helical turns. The transition free surgery is approximately kcal/mol. (iii) The chemical modification pattern of the resulting structure suggests that the purine strand folds back and associates with the pyrimidine strand, forming a novel intramolecular triplex structure consisting of d(A.A.T) base triplets. A model for the triplex conformation is proposed and its thermodynamic properties are analyzed by statistical mechanics.