We describe the helical structure of Bordetella pertussis fimbriae of serotype 3/6 as determined to a resolution of similar to 2.5 nm by three-dimensional reconstruction of negatively stained electron micrographs, The fimbria has a distinctly polar structure whose axial repeat of 13 nm contains five copies of the fim3 gene product (22 kDa) in two complete turns, These subunits are connected by interactions along the fimbrial backbone which, unlike other classes of bacterial fimbriae, has no axial channel, Its outer diameter is similar to 5.7 nm, and the most pronounced feature is a radially protruding domain that gives the fimbria its characteristic serrated appearance. Serotype 2 fimbriae, composed of the fim2 subunit which is 60% homologous with fim3, have essentially the same quaternary structure, These observations are discussed in relation to fimbrial phase variation and structure-based classification of fimbriae/pili. (C) 1996 Academic Press, Inc.