An unusual Actinomyces-like bacterium originating from a pig with mastitis was subjected to a polyphasic taxonomic investigation. The morphological and biochemical characteristics of the organism were consistent with its preliminary assignment to the genus Actinomyces but it did not appear to correspond to any recognized species. PAGE analysis of whole-cell proteins confirmed the phenotypic distinctiveness of the bacterium and 16S rRNA gene sequence analysis demonstrated that it represents a hitherto unknown sub-line amongst a cluster of Actinomyces species which embraces Actinomyces canis, Actinomyces georgiae, Actinomyces hyovaginalis, Actinomyces meyeri, Actinomyces odontolyticus, Actinomyces radingae and Actinomyces turicensis. Based on phylogenetic and phenotypic evidence, it is proposed that the unknown bacterium isolated from pig mastitis be classified as Actinomyces suimastitidis sp. nov. The type strain of Actinomyces suimastitidis is CCUG 39276 T (l CIP 106779 T ).