Raman spectra have been observed of two different poly[d(A-T)]·poly[d(A-T)] fibers, whose X-ray diffractions were confirmed to be purely of A and B forms. A number of spectral differences were found between the A and B forms of this DNA duplex, and they were ascribed to local conformational differences in the adenosine, thymidine and phosphodiester portions. The ascription was made on the basis of a separate series of Raman examinations on six crystals involving adenosine or thymidine, and fifteen other nucleotide crystals, whose structures are all known by previous crystallographic works. By taking these structure-spectrum correlations thus obtained into account, a Raman spectroscopic investigation was made of a few double-helical DNAs in aqueous solutions. It has been concluded that both poly[d(A-T)]·poly[d(A-T)] and poly(dA)·poly(dT) have a C2′endo-anti adenosine, C2′endo-anti thymidine, a b-type mainchain (β=160±15°, γ=45±15°, δ=140±10°) and an a2-type mainchain (β=210±10°, γ=45±15°, δ=140±10°) not only in low-salt medium but also in 6.6 M CsF solutions, where β, γ and δ are the torsion angles around O5′-C5′, C5′-C4′ and C4′-C3′ axes, respectively. Poly(rA)·poly(dT), on the other hand, was considered to have a heteronomous duplex structure, in which the poly(rA) strand has a C3′endo-anti adenosine and a1-type mainchain (β=175±25°, γ=45±15°, δ=80±10°) whereas the poly(dT) strand has a C2′endo-anti thymidine and b-type mainchain. © 1986.