Fifty-four carboxydotrophic actinomycetes isolated from soils and composts were compared through 119 unit characters with representative mesophilic and thermophilic streptomycetes. The data were examined using the Jaccard, pattern and simple matching coefficients and clustering achieved using the unweighted pair group method with arithmetic averages algorithm. Acceptable cophenetic correlation and test error values allowed confidence to be placed in the resultant numerical taxonomies. The carboxydotrophic actinomycetes, which were distinct from cluster-groups corresponding to the mesophilic and thermophilic streptomycetes, formed two major cluster-groups the members of which were examined for the presence of diagnostic chemical markers. All but two of the carboxydotrophic actinomycetes had a profile of chemical properties consistent with their assignment to the genus Streptomyces. Quantitative fatty acid data were examined using the SIMCA package and the two statistically significant groups obtained corresponded with the cluster-groups circumscribed in the numerical phenetic analysis. Members of the two groups were also distinguished on the basis of their phospholipid composition. The two strains that contained meso-as opposed to LL-diaminopimelic acid in their peptidoglycan also showed a distinct chemotaxonomic profile. It was concluded that the carboxydotrophic actinomycetes form a novel and taxonomically diverse group.