Summary Thermophilic species of nine genera of Actinomycetales were characterized by morphology, biochemical markers, physiology and phage-typing. The results of this study led to a key for the identification of the genera Actinomadura, Faenia, Microbispora, Micropolyspora, Pseudono cardia, Saccharomonospora, Saccharopolyspora and Thermomonospora . On the basis of our biochemical data, the following species, most of them not included in the Approved Lists of Bacterial Names, are described: Actinomadura angiospora comb. nov. (formerly Micropolyspora angiospora ), Actinomadura rubrobrunea nom. rev., comb. nov. (containing the two former species ‘ Excellospora rubrobrunea ” and ‘ Excellospora viridinigra ’), Saccharomonospora caesia nom. rev., comb. nov. (formerly ‘ Micropolyspora caesia ’), Saccharomonospora internatus comb. nov. (formerly Micropolyspora internatus ) and Thermopolyspora flexuosa nom. rev., comb. nov. (formerly ‘ Actinomadura flexuosa ’). In the case of Microbispora, Saccharomonospora and Thermomonospora further studies are needed for species identification.