Type 4 fimbriae are important colonization factors in Pseudomonas aeruginosa and other pathogens that mediate attachment to epithelial cells of the host. They are also responsible for a form of translocation termed 'twitching motility' and are implicated in the susceptibility to fimbrial-specific bacteriophage. Analysis of a transposon mutant which lacks functional fimbriae has identified a new gene which is required for fimbrial biogenesis. This gene, termed pilV, is located on chromosomal SpeI fragment E, 2 kb downstream of the previously characterized pilSR genes involved in transcriptional activation of the fimbrial subunit gene. The pilV gene encodes a 20 kDa membrane-located protein with considerable amino-terminal homology to the type 4 consensus pre-pilin leader sequence, suggesting that it is processed by a leader peptidase. Site-directed mutagenesis has shown that PilV requires such cleavage to be functional. PilV also exhibits close similarity to a group of proteins involved in extracellular protein secretion from a number of Gram-negative bacteria, suggesting that the biogenesis of type 4 fimbriae may have a similar basis.