Mayaro (MAY) virus is a member of the genus Alphavirus in the family Togaviridae. Alphaviruses are distributed throughout the world and cause a wide range of diseases in humans and animals. Here, we determined the complete nucleotide sequence of MAY from a viral strain isolated from a French Guianese patient. The deduced MAY genome was 11,429 nucleotides in length, excluding the 5' cap nucleotide and 3' poly(A) tail. Nucleotide and amino acid homologies, as well as phylogenetic analyses of the obtained sequence confirmed that MAY is not a recombinant virus and belongs to the Semliki Forest complex according to the antigenic complex classification. Furthermore, analyses based on the E1 region revealed that MAY is closely related to Una virus, the only other South American virus clustering with the Old World viruses. On the basis of our results and of the alphaviruses diversity and pathogenicity, we suggest that alphaviruses may have an Old World origin.