The taxonomic status of 'Streptomyces avermitilis' strain MA-4680 was established using a polyphasic approach. Strain MA-4680 formed a distinct phyletic line in the 16S rDNA streptomycete tree, and it was evident from the almost complete 16S rDNA sequence data that it was most closely related to Streptomyces cinnabarinus, Streptomyces griseochromogenes, Streptomyces resistomycificus and Streptomyces viridochromogenes. However, strain MA-4680 was readily distinguished from the type strains of these species by using a range of phenotypic properties, notably morphological and pigmentation features. The combined genotypic and phenotypic datasets indicate that the organism forms a recognizable centre of variation within the genus Streptomyces. It is proposed that 'Streptomyces avermitilis' be formally recognized as a species of Streptomyces. The type strain is MA-4680(T) (ATCC 31267(T) = NCIMB 12804(T0 = NRRL 8165(T)).