The morphology, chemotaxonomy, and cultural and physiological characteristics were examined on the five strains of actinomycetes which produce antiviral antibiotics, fluvirucin congeners. All strains have meso-2,6-diaminopimelic acid in the cell wall. Four strains, Q464-31, L407-5, R359-5 and R516-16, belong to the maduromycetes since they have madurose in the whole cell. The remaining one strain, R869-90, has rhamnose but no madurose, and is a nocardioform actinomycete. These five strains were classified and designated as follows: Strain Q464-31 (fluvirucin A1 producer): Microtetraspora tyrrhenii sp. nov. (Actinomadura pusilla group). Strain L407-5 (fluvirucin B2 producer): A maduromycete. Strain R359-5 (fluvirucin B1 producer): Microtetraspora pusilla (Actinomadura pusilla group). Strain R869-90 (fluvirucin A2 producer): Saccharothrix mutabilis. Strain R516-16 (fluvirucins B2, B3, B4 and B5 producer): A maduromycete.