Insertion mutants of recombinant plasmids encoding type 1 fimbriae of four genera of enteric bacteria were used to detect genetic complementation. After transformation by pairs of plasmids, double transformants were screened for their ability to express type 1 fimbriae. Complementation was observed between genes derived from the same genus but was absent with chimeric molecules carrying genetic information from two different genera. The results indicate that diffusible gene products of the fim cluster are necessary for phenotypic expression of type 1 fimbriae.