Herpesviruses are large double-stranded DNA viruses which infect vertebrates from fish to man. Convincing genetic relationships between herpesviruses that infect higher vertebrates (birds and mammals) have been demonstrated previously by comparing proteins predicted from DNA sequences and have been interpreted as a result of evolution from a common ancestor. In order to evaluate how herpesviruses of lower vertebrates fit into this scheme, the 134,226-bp genome of channel catfish virus was sequenced. Genetic comparisons indicate that a separate evolutionary origin for this virus must be considered. The findings impact upon current perceptions of herpesvirus evolution and gene function.