Norwalk virus (NV) is the prototype human calicivirus, and causes epidemic outbreaks of acute gastroenteritis. The sequence and predicted genome organization of NV and a NV-like virus [Southampton virus (SHV)] suggested they are similar viruses at the nucleotide and amino acid level, although SHV was reported to be antigenically distinct from NV. A recent review described the discovery of an additional 12 nucleotides at the 5' end of SHV and prompted us to investigate the possibility of additional nucleotides at the 5' end of the NV genome. The results obtained by homopolymeric tailing of NV cDNA with dCTP and dATP showed 12 additional nucleotides also are present on the NV genomic RNA. These data are important with respect to the biology of the virus, and make the genome sequence of NV complete.