Molecular studies have been carried out on Tomato mild mottle virus (TMMV) and 2322 nucleotides of the aphid-transmitted virus have been sequenced. This translates to a 2187 nucleotide open reading frame and a 135 nucleotide untranslated region (UTR) followed by a poly A tail. The sequence is consistent with the NIb and coat protein regions of a member of the Potyviridae family of viruses. Sequence comparisons show this virus to have no close relationship with any previously sequenced aphid-transmitted virus; the highest identity was found with the whitefly-transmitted Ipomoviruses. The vector of transmission and the low level of sequence identity with other aphid-transmitted viruses suggest TMMV does not belong to any existing genus of the Potyviridae family.