A total of 30 iridoviruses collected from Australia, South-East Asia, North America, South America and Europe were characterised. With the exception of the South-East Asian iridoviruses all viruses were found to belong to the genus Ranavirus. All viruses, except those originating from South-East Asia, cross-reacted with antisera against epizootic haematopoietic necrosis virus (EHNV). Viruses or virus-infected cells were examined using electron microscopy, SDS PAGE, restriction endonuclease (RE) digestion, DNA hybridisation, and DNA sequencing. Data from RE digestion of genomic DNA, and from the sequencing of PCR products indicated that the viruses generally grouped according to their geographic and taxonomic (i.e. amphibian or fish) origin. The one exception to this was the viruses from the United Kingdom that grouped with the North American ranaviruses. The differences between specified genomic regions were small. To assess the validity of the differences in sequence homology, similar studies were performed with different isolates from two viruses (EHNV and Guatopo virus (GV), collected from different animals at different locations and time). The sequence data showed complete homology for the isolates for any one virus over the 200 and 586 bp regions examined. Collectively, the data showed that the coding region for the major coat protein (MCP) is stable for any one species (e.g. EHNV).