Aggregatibacter actinomycetemcomitans is usually isolated from the oral cavity where it is associated with active periodontitis. The species can be divided into six serotypes (a-f) according to their surface carbohydrate antigens. However, some clinical isolates cannot be grouped within these six serotypes. Gram-negative, facultative anaerobic, catalase-positive coccobacilli were isolated from a patient with periodontitis and identified by employing genetic, biochemical and serological analyses. Phenotypic data identified the isolate as A. actinomycetemcomitans. Serotype-specific polysaccharide antigen from the isolate was untypeable by immunodiffusion testing in comparison with reference A. actinomycetemcomitans serotype a to f strains. Biofilm formation by the isolate was strong but cytotoxic activity was low. Gas chromatography/mass spectroscopy analysis of partially methylated alditol acetates from surface polysaccharide showed the presence of 2,4-di-O-methyl-rhamnose and 2,3,6-tri-O-methyl-glucose, with a 1 : 1 m ratio. The (1)H- and (13)C-nuclear magnetic resonance spectra of the antigen showed that both constituent glycoses had alpha-anomeric configuration. It is proposed that the untyped strain is a new A. actinomycetemcomitans serotype, designated serotype g.