We have analyzed and compared the amino acid sequences of the type 4 fimbrial subunits from Pseudomonas aeruginosa. Moraxella bovis, M. nonliquefaciens, Bacteroides nodosus, Neisseria gonorrhoeae, and N. meningitidis. We propose a consensus sequence for the highly conserved amino-terminal regions of these proteins. In the variable regions, a domain corresponding to an epitope common to N. gonorrhoeae and N. meningitidis fimbriae is conserved, both in sequence and in environment, in fimbrial subunits from B. nodosus. The subunits from M. bovis and P. aeruginosa do not show any homologies to this sequence. In all of the subunits, the carboxy-terminal half of the molecule consists of a series of fairly hydrophobic domains. The last three domains, two of which include the cysteines of the disulfide bridge in N. gonorrhoeae, P. aeruginosa, and M. bovis, are more or less conserved in sequence in all of the proteins including that of B. nodosus. We propose that these conserved hydrophobic regions, which have the potential to form a series of beta-sheets, form a structural framework around which more variable hydrophilic sequences determining immunological profile are arranged. The evolutionary relationships of the contemporary proteins and the distribution of type 4 fimbriae are also discussed.