A systematic phylogenetic analysis of the genus Actinomyces was performed. The 16S rRNA gene sequences of 13 Actinomyces species, an unnamed Actinomyces strain (ATCC 49338), and an Actinomyces-like isolate from sea mammals were determined. Comparative sequence analysis with closely related taxa revealed phylogenetic diversity and internal structure within the genus Actinomyces. In addition, some members of other genera (viz., the genera Arcanobacterium, Mobiluncus, and Rothia) were shown to be phylogenetically intermixed with the Actinomyces species. It was evident from both distance and tree topology considerations that the genus Actinomyces is in urgent need of taxonomic revision and requires subdivision into several genera. Based on the results of the present study it is proposed that Actinomyces bernardiae and Actinomyces pyogenes be assigned to the genus Arcanobacterium as Arcanobacterium bernardiae comb. nov. and Arcanobacterium pyogenes comb. nov., respectively. In addition, a new species, Arcanobacterium phocae, is proposed for Actinomyces-like bacteria isolated from seals.