The X-ray crystal structure of the decamer C-G-A-T-A-T-A-T-C-G has been solved with two contrasting cations, Ca2+ and Mg2+. Crystals with calcium are space group P2(1)2(1)2(1), cell dimensions a = 38.76 A, b = 40.06 A, and c = 33.73 A, and diffract to 1.7-A resolution. Crystals with magnesium have the same space group, cell dimensions a = 38.69 A, b = 39.56 A, and c = 33.64 A, and diffract to 2.0 A. Their structures were solved independently by molecular replacement, beginning with idealized Arnott B-DNA geometry. The calcium structure refined to R = 17.8% for the 3683 reflections greater than 2 sigma, with 404 DNA atoms, 95 solvent peaks, and 1 Ca(H2O)7(2+) ion. The magnesium structure refined to R = 16.5% for the 1852 reflections greater than 2 sigma, with 404 DNA atoms, 62 solvent peaks, and 1 Mg(H2O)6(2+) ion. The two structures are virtually identical and are isostructural with C-G-A-T-C-G-A-T-C-G [Grzeskowiak et al. (1991) J. Biol. Chem. 266, 8861-8883] and C-G-A-T-T-A-A-T-C-G [Quintana et al. (1992) J. Mol. Biol. 225, 375-395]. Comparison of C-G-A-T-A-T-A-T-C-G with C-G-C-A-T-A-T-A-T-G-C-G [Yoon et al. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 6332-6336] shows that the expected alternation of twist angles is found in the central A-T-A-T-A-T region of the decamer (A-T small, T-A large), but the minor groove remains wide at the center, rather than narrow. Minor groove narrowing is produced, in these two structures, not by overwinding of the helix, but by an increase in base pair propeller. This analysis confirms the concept that poly(dA-dT).poly(dA-dT) is polymorphous, with different local conformations possible in different local environments.