A new method for determining nucleotide sequences in DNA is described. It is similar to the "plus and minus" method Sanger, F. & Coulson, A. R. (1975) J. Mol. Biol. 94, 441-448 but makes use of the 2',3'-dideoxy and arabinonucleoside analogues of the normal deoxynucleoside triphosphates, which act as specific chain-terminating inhibitors of DNA polymerase. The technique has been applied to the DNA of bacteriophage varphiX174 and is more rapid and more accurate than either the plus or the minus method.