Gram-negative purple bacteria possess pairs of extracytoplasmic, ATP-independent, fimbrium-specific chaperone proteins which cooperatively function in the assembly of this extracellular organelle. The two non-homologous families of these proteins have been termed "Fimbrial chaperone family no. 1" (FCF1) and "Fimbrial chaperone family no. 2" (FCF2). The eleven sequenced or partially sequenced members of each of these two protein families were analysed. Their sequences were multiply aligned, and average similarity and hydropathy plots were generated. Statistical analyses of the sequences revealed that the short FCF1 proteins (of about 240 residues) have been better conserved through evolutionary time than have the much larger FCF2 proteins (of about 830 residues). Moreover, the N-terminal thirds of the FCF2 proteins are better conserved than the central or C-terminal thirds of these proteins. Phylogenetic tree construction revealed that, in general, the two proteins which cooperate in the assembly of a particular fimbrial type have similar positions on their respective phylogenetic trees, suggesting that the two proteins evolved in parallel as a functional unit. Two exceptions were noted, however. In one case, a hybrid protein appears to have arisen, possibly by genetic recombination. In another case, the two proteins of a particular pair may have evolved separately and come together late in the evolutionary process to provide their cooperative function.