The Fifth Report of the International Committee on Taxonomy of Viruses (ICTV) has been organized in the same way as the previous ones. Yet, it encompasses many more families and groups of viruses than previous reports. Included are diagrams by their type of host, tables listing the viruses by alphabetical order as well as by their host type and their type of nucleic acid, and keys of family and group identification. The officers and members of the ICTV study groups from 1984 to 1990 are listed, as are the curent ICTV statutes and rules of nomenclature. Information on the format for submission of new taxonomic proposals to the ICTV is also provided, and indices of virus, taxonomic and authors' names are provided at the end of the report.