Summary The phylogenetic structure of the genus Streptosporangium was determined by comparative 16S rDNA sequence analysis of 26 Streptosporangium strains, including the type strains of all validly-described Streptosporangium species. Streptosporangium forms a phylogenetically coherent genus and appears to be most closely related to members of the genus Streptomyces . Ten of the thirteen type strains investigated were shown to be highly related, forming strain cluster I, with a substructure of five lineages. This cluster includes the type strain of the type species of the genus, S. roseum DSM 43021 T . Strain cluster II is composed of the other three type strains, i. e., S. corrugatum DSM 43316 T , S. claviforme DSM 44127 T and S. viridialbum DSM 43181 T . Five non-type strains of validly described Streptosporangium species were also investigated; four of these were highly related to their respective type strains, while the other was more closely related to the type strain of another species. Of seven non-validly described taxa studied, four were highly related to S. roseum DSM 43021 T , belonging to strain cluster I, one forms a potential sixth subcluster of cluster I, one belongs to cluster II, and S. “sibiricum” fell outside the radiation of Streptosporangium . Characteristic patterns of signature oligonucleotides differentiate members of Streptosporangium from members of Streptomyces , cluster I organisms from those of cluster II, and the strains of the different subclusters of cluster I from each other. These signatures could be used for future design of specific oligonucleotide probes or primers.