A new design for DNA bases and base pairs is described in which the pyrimidine bases are widened by naphtho-homologation. Two naphtho-homologated deoxyribosides, dyyT (1) and dyyC (2), were synthesized and could be incorporated into oligonucleotides as suitably protected phosphoramidite derivatives. The deoxyribosides were found to be fluorescent, with emission maxima at 446 and 433 nm, respectively. Studies with single substitutions of 1 and 2 in the natural DNA context revealed exceptionally strong base stacking propensity for both. Sequences containing multiple substitutions of 1 and 2 paired opposite adenine and guanine were subsequently mixed and studied by several analytical methods. Data from UV mixing experiments, FRET measurements, fluorescence quenching experiments, and hybridizations on beads suggest that complementary "doublewide DNA" (yyDNA) strands may self-assemble into helical complexes with 1:1 stoichiometry. Data from thermal denaturation plots and CD spectra were less conclusive. Control experiments in one sequence context gave evidence that yyDNA helices, if formed, are preferentially antiparallel and are sequence selective. Hypothesized base pairing schemes are analogous to Watson-Crick pairing, but with glycosidic C1'-C1' distances widened by over 45%, to ca. 15.2 A. The possible self-assembly of the double-wide DNA helix establishes a new limit for the size of information-encoding, DNA-like molecules, and the fluorescence of yyDNA bases suggests uses as reporters in monomeric and oligomeric forms.