The complete nucleotide sequence of the genome of coxsackievirus A9 (CAV-9) has been determined from cDNA cloned in Escherichia coli. Excluding the 3' poly(A) stretch, the RNA genome is 7452 nucleotides long and encodes a single polyprotein of 2201 amino acids. Comparison of the nucleotide and predicted amino acid sequences with those of the coxsackieviruses B1, B3 and B4 reveals a surprising degree of homology, with overall amino acid homologies of 86.9%, 86.2% and 87.0%, respectively. In contrast, there is much less homology to another coxsackie A virus, CAV-21, 60.4% overall amino acid homology. This demonstrates the high degree of diversity within the CAV group and indicates that the current classification does not directly correlate with molecular genetic properties. One major feature of CAV-9 is an insertion, relative to all other enteroviruses sequenced to date, which is located at the C terminus of VP1, and includes an arginine-glycine-aspartic acid tripeptide. Such sequences in a number of other proteins are known to have activity in promoting attachment to cell receptors and the implications for CAV-9 receptor binding are discussed.