The Hytrosaviridae comprises a family of dsDNA viruses with a circular genome of 120-190 kb p. They are exclusively associated with Diptera, such as the tsetse fly, the house fly and the Narcissus bulb fly. Hytrosaviruses cause a very unique pathology including hypertrophy of salivary glands as well as testicular and ovarian malformation. On the other hand these viruses share a significant number of gene homologues with other dsDNA viruses, esp. baculoviruses and nudiviruses. These gene homologues include twelve so-called baculovirus core genes involved in transcription, DNA replication and the infection process. Most strikingly, the Musca domestica salivary gland hypertrophy virus (MdSGHV) encodes a homologue of a polyhedrin/granulin gene of Alpha-, Beta-, Gammabaculoviruses. Hence, it is proposed that hytrosaviruses are phylogenetically related to baculoviruses but evolved in a very close association with their dipteran hosts.