The taxonomic status of six strains received as Streptomyces hygroscopicus was established using a polyphasic approach. The organisms had phenotypic and morphological properties consistent with their classification in the genus Streptomyces. Almost complete 16S rRNA gene sequences of the strains were determined and aligned with corresponding sequences of representatives of the genus Streptomyces and phylogenetic trees were inferred using four tree-making algorithms. All of the strains fell within the Streptomyces violaceusniger 16S rRNA gene clade and four of them were considered to merit species status using a combination of genotypic and phenotypic data. These organisms were designated Streptomyces himastatinicus sp. nov. (type strain ATCC 53653(T)=DSM 41914(T)), Streptomyces mordarskii sp. nov. (type strain NRRL B-1346(T)=DSM 40771(T)), Streptomyces rapamycinicus sp. nov. (type strain NRRL 5491(T)=ATCC 29253(T)) and Streptomyces ruanii sp. nov. (type strain ISP 5276(T)=DSM 40276(T)). DNA-DNA relatedness and phenotypic data showed that the two remaining strains, 'Streptomyces hygroscopicus subsp. enhygrus' NRRL 3664 and Streptomyces hygroscopicus subsp. hygroscopicus NRRL 3111 belong to the same species as the type strain of Streptomyces rutgersensis subsp. castelarensis. This taxon is designated Streptomyces castelarensis sp. nov., comb. nov. (type strain ATCC 15191(T)=DSM 40830(T)) following the proposal that Streptomyces rutgersensis subsp. rutgersensis is a heterotypic synonym of Streptomyces albidoflavus.