Complete 5S ribosomal RNA sequences were obtained for four acidophilic actinomycetes, seven neutrophilic streptomycetes and a strain of Streptoverticillium baldaccii. All of the organisms contained RNAs belonging to the 120 nucleotide type. An evolutionary tree was generated after combining the test data with results from similar studies on representative Gram-positive bacteria. The acidophilic, neutrotolerant and neutrophilic actinomycetes were recovered in a distinct cluster that was equated with the genus Streptomyces. The sequence data support the view that the genera Chainia, Elytrosporangium, Kitasatoa and Microellobosporia should be considered as synonyms of the genus Streptomyces. The recovery of the Streptoverticillium baldaccii strain on the fringe of the Streptomyces cluster is also consistent with current trends in the taxonomy of these organisms. Further work is needed to determine the taxonomic status of the two streptomycete subgroups that comprised the streptomycete cluster.