X-ray diffraction in fibres revealed that the calcium salt of poly(dA).poly(dT) is a 10-fold double helix with a pitch of 3.23 nm. The opposite sugar-phosphate chains in the refined model are characterized by a complete conformational equivalence and contain sugars in a conformation close to C2'-endo. As a result a new model of the sodium salt of poly(dA).poly(dT) has been constructed, which is different from the Heteronomous DNA proposed earlier (S. Arnott et al., Nucl. Acids Res. 11, 4141 (1983)). The new model of Na-poly(dA).poly(dT) has conformationally similar opposite chains; it is a structure of the B-type, rather like that of Ca-poly(dA).poly(dT).