This study is the first to define the S. violaceoruber species group and shows that a number of named species including S. coelicolor, S. lividans and S. griseoruber, amongst others, to be the same species, forming a tight clade in a multigene analysis. Comparisons were made to strains of S. grisells to determine the overall similarity in the context of studying horizontal gene transfer of the streptomycin biosynthetic gene cluster. All type strains grouping with S. violaceorllber-are considered to be synonyms of this species. Phylogenetic analysis was done using the house-keeping genes 16S rRNA, gyrB, recA and tlpB. Genes for adaptive functions were also used and these included those for chiF (chitinase) and strA (streptomycin resistance), which provide an adaptive function to the organisms. Soil isolates identified as S. platensis, S. limoslls and S. violaceorllber were found to contain either partial or the entire streptomycin gene clusters. In isolates closely related to S. grisells the location of the streptomycin cluster was the same as that in the type strain S. grisells DSM 40236. Analysis of CR20 (8. platensis), 651 (S. limoslls) and ASB37 (S. violaceoruber) indicated that the streptomycin cluster was in a different position compared to S. griseus DSM 40236. The exact location of the streptomycin cluster on the chromosomes of these strains has yet to be elucidated. Algorithms used to determine gene histories were applied to the gene sequences of the house-keeping genes and cMF and strA and analysis confirmed· gene transfer ofstrA from a putative S. griseus source to soil isolates identified as being phylogenetically distinct from S. grisells. strA gene sequences demonstrated this gene to have a significantly different gene history from the house-keeping genes and that it was evolving at a much faster rate. This study also placed chiF with the house-keeping genes and confirmed previous findings that chiF is an ancient gene transfer event into the Streptomyces genome and which has ameliorated over time. The statistical analysis of the phylogenetic trees was able to identify a core set of genes that can be used for phylogeny and that have not been subjected to transfer events. S. coelicolor A3(2)(W) and ASB37 were found to be closely related to S. coelicolor M145, however these strains differed in their phenotypic properties. Comparative genomic hybridisation between S. coelicolor A3(2)(W) and ASB37 against S. coelicolor M145 have revealed extensive genetic differences which correlated with the observed phenotypes.