The serotype f antigen of Streptococcus mutans has been described as a rhamnose-glucose polysaccharide associated with the bacterial cell wall. In this study, the structure of serotype f polysaccharide was examined by analyses of the methylated derivatives of the antigen and the periodate-oxidized antigen. Methylated derivatives were characterized with a gas chromatograph-mass spectrometer. The polysaccharide appeared to have a backbone of alternating 1,3- and 1,2,3-linked rhamnose units. Branching occurred at the 3-position of the 1,2,3-linked rhamnose. Side chains were composed of terminal alpha-linked glucose units. A small proportion of longer side chains containing 1,2- and 1,6-linked glucose units were noted in some preparations; however, these determinants were not reactive with serotype f antisera.