Many oral streptococci express large surface structures (fibrils and fimbriae) that have been implicated in adhesion of bacteria to oral surfaces. Streptococcus gordonii DL1 short peritrichous fibrils (61 nm long) are composed of cell-wall-anchored CshA adhesin (259 kDa). ELISA studies have previously shown that proteins immunologically related to CshA are expressed widely by other mitis group streptococci (including S. mitis, S. sanguis, and S. oralis) although it is not known whether the CshA-like proteins are structural components of fibrils in these organisms. To address this, we investigated the surface structures of several mitis group streptococci that express CshA. The identity of all strains was confirmed by 16s rDNA sequencing. Short fibrils (51-67 nm long) were present on all strains examined and fibril length was characteristic for each strain. Immunoelectron microscopy was performed using antiserum specific for the N-terminal portion of S. gordonii DL1 CshA (N-CshA antiserum). For most strains, the gold label was located towards the tips of short fibrils, consistent with S. gordonii DL1 observations and with the molecular architecture of CshA. For S. gordonii MJ2, however, gold particles were significantly closer to the cell surface (25.5 7.2 nm), corresponding to the mid-point of short fibrils (length 50.6 9.4 nm). Additionally, four strains of S. gordonii did not react in ELISA with N-CshA antiserum but reacted strongly with antiserum to the C-terminal region of CshA. The results indicate that CshA may comprise short fibrils on a range of oral streptococcal species, although some strains may express non-conventional CshA molecules that nevertheless form fibril structures.