Sequence results are presented for a 3745-nucleotide region at the 5' end of the dengue type-1 virus (DEN-1) genome. The strain characterized is a Western Pacific isolate from Nauru Island. The sequenced region contains the beginning of a continuous open reading frame which specifies the capsid (C), membrane (M), and envelope (E) structural proteins and the nonstructural protein NS1. The sequences are compared with corresponding segments for seven other flaviviruses, including two of the three remaining dengue serotypes, DEN-2 and DEN-4. The results show the DEN-1 genome size and organization to be similar to those of other characterized flaviviruses and that major features of the individual proteins are conserved. It is of special interest that comparisons of the E glycoprotein sequences between the dengue serotypes (DEN-1, -2, -4) reveal only moderately greater sequence relatedness (63-68%) than occurs in comparisons of DEN-1 with five other flaviviruses (46-54%). For the other structural proteins, C and M, the relatedness values are 59-74% for comparisons between DEN-1 and the other dengue serotypes and 31-45% for comparisons between DEN-1 and the five other flaviviruses.