This study attempted to elucidate the role of fimbriae in the adherence of B. catarrhalis to human oropharyngeal epithelial cells. Antifimbrial immune serum was prepared by immunization of rabbit with whole fimbriated bacteria, and adsorption of the serum with a nonfimbriated B. catarrhalis strain. After pretreatment with the antifimbrial antiserum, the adherence of fimbriated B. catarrhalis to human epithelial cells was significantly decreased (p < 0.05). The adherence was also significantly (p < 0.001) decreased by trypsin treatment. Electron microscopy revealed destruction of fimbriae after trypsin treatment. These observations suggest that fimbriae are involved in the adherence of B. catarrhalis to epithelial cells.