We describe the structure in aqueous solution of an extended-size DNA-like duplex with base pairs that are similar to2.4 Angstrom longer than those of DNA. Deoxy-lin-benzoadenosine (dxA) was employed as a CIA analogue to form hydrogen-bonded base pairs with dT. The 10mer self-complementary extended oligodeoxynucleotide 5'-d(xATxAxATxATTxAT) forms a much more thermodynamically stable duplex than the corresponding DNA sequence, 5'-d(ATAATATTAT). NMR studies show that this extended DNA (xDNA) retains many features of natural B-form DNA, but with a few structural alterations due to its increased helical diameter. The results give insight into the structural plasticity of the natural DNA backbone and lend insight into the evolutionary origins of the natural base pairs. Finally, this structural study confirms the hypothesis that extended nucleobase analogues can form stable DNA-like structures, suggesting that alternative genetic systems might be viable for storage and transfer of genetic information.