An exceptionally strong quadruply hydrogen-bonded complex is formed between 2,7-diamido-1,8-naphthyridine 3 (DAN) and the butylurea of guanosine 6 (UG) in chloroform. The UG unit can be prepared in four steps from guanosine on a 10 g scale in excellent yields without chromatographic purification. The association constant (K-assoc approximate to 5 x 10(7) m(-1)) for the UG(.)DAN complex determined by fluorescence energy transfer from the naphthyridine unit of 3 to coumarin 343 covalently linked UG (18) is among the highest reported for a neutral DNA base-pair analogue. The weak self-association of DAN (K-dimer < 10 m(-1)) and UG (K-dimer ca. 200-300 m(-1)) means that the UG-DAN complex forms with unparalleled fidelity.