Ten strains of thermophilic actinomycetes were isolated from waste and mushroom composts, as well as from the air of compost plants and a refuse incineration plant in Germany. These organisms produce white aerial mycelia and form hyphae with so-called pseudosporangia that fragment into rod-like structures. The organisms have type III cell walls (meso-diaminopimelic acid and whole-cell sugar type C), the phospholipid type is type PII, and mycolic acids are not present. The major menaquinone is MK-9(H4), and the fatty acids are mainly iso- and anteiso-branched fatty acids, hydroxy fatty acids, and 10-methyl-branched fatty acids. The guanine-plus-cytosine content of the DNA is 69 to 73 mol%. The chemotaxonomic markers (especially whole-cell sugar type C) and 16S ribosomal DNA sequence data indicated that these organisms represent a new genus of the order Actinomycetales, for which the name Thermocrispum is proposed. On the basis of phylogenetic and phenotypic data, this new genus is closely related to members of the family Pseudonocardiaceae and related taxa and contains two species: Thermocrispum municipale sp. nov. and Thermocrispum agreste sp. nov. The type species of the genus is T. municipale, with type strain MKD 35 (= DSM 44069), and the type strain of T. agreste is CHB 77 (= DSM 44070).