A micro-organism resembling members of the genus Dermatophilus, strain W254 T , which was isolated from the submandibular lymph node of a pig, and an additional 16 strains isolated from swine tonsils, were studied to establish their taxonomic status. Although all 17 strains were isolated anaerobically under an atmosphere of 100 % CO 2 , all of them were aerotolerant anaerobes. The micro-organisms showed at least five cellular morphologies: (i) a radially protrusive thallus, which proliferated into tuber-like cells; (ii) segmentation in both tubers and thallus followed by multilocule formation, (iii) development of coccoid forms in the locules; (iv) a change from the coccoid forms to zoospores; (v) resting cells, which were able to develop into protrusive thalli again. The microorganisms were positive for nitrate reduction, but negative for catalase, indole production, hydrolysis of urea and gelatin liquefaction. Milk was not decomposed and none of the strains was haemolytic. A total of 16 compounds, including glucose, were utilized as sole carbon sources and seven compounds, including L-arabinose, were not utilized. Three out of the 17 strains were subjected to further studies. The micro-organisms had meso-diaminopimelic acid in their peptidoglycan and galactose, glucose, madurose and a trace of mannose in their whole-cell sugar patterns. The major phospholipids were phosphatidylglycerol, diphosphatidylglycerol and phosphatidylinositol.Abbreviations: DAP, diaminopimelic acid; SEM, scanning electron microscopy; TEM, transmission electron microscopy. The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA gene sequences of Tonsilliphilus suis W254 T , HT1-19 and HT5-225 are AB096084, AB096085 and AB602380, respectively. A supplementary figure and a supplementary table are available with the online version of this paper.