Iridoviruses have been increasingly associated with localized outbreaks of severe disease among diverse fish species. Systemic infections were seen in sheatfish and catfish in Europe, among white sturgeon in the United States, in rainbow trout and other species in Australia, and among commercially important marine fish in south-east Asia. Nine novel piscine isolates, along with two each from reptiles and frogs, were characterized by examination of protein synthesis in vivo, restriction endonuclease profiles, and sequence analysis of the major capsid protein gene. These studies clearly demonstrated that piscine iridoviruses were more similar to frog virus 3 (genus ranavirus) than to the previously-described fish iridovirus lymphocystis disease virus (genus lymphocystivirus). This work indicates that the genus ranavirus includes viruses infecting all three classes of cold-blooded vertebrates and suggests that surveys of fish viruses may need to include neighboring amphibian and reptilian populations. Furthermore, the ability of these viruses to replicate in channel catfish fibroblast cell lines suggests that it should be possible to monitor natural and/or acquired immunity directed against iridovirus-infected targets. This not only expands the panel of viruses which can be used to monitor anti-viral immune responses in fish, but may also provide valuable insights into understanding and controlling iridovirus-mediated disease.