The major outer membrane protein, FomA, of the Gram-negative human oral pathogen Fusobacterium nucleatum functions as a porin and is assumed to act as a receptor protein in coaggregation with other oral pathogenic bacteria such as Streptococcus sanguis and Porphyromonas gingivalis. We describe here the cloning of fomA from F. nucleatum in E. coli. Using pGEM3Zf(+), three recombinant plasmids were carrying parts of the fomA gene, but none of these contained regions upstream of the coding sequence. From these plasmids a clone was constructed which contained the whole fomA gene. The ATCC 10953 fomA gene was cloned under the phosphate limitation-inducible phoE promoter, using a vector derived from pACYC184. The protein was found to be incorporated into the outer membrane of the host in an apparently normal manner, as judged by heat-modifiability, trypsin-accessibility, and accessibility to antibodies to the protein in a whole cell enzyme-linked immunosorbent assay. The cloned FomA was found to exhibit pore-forming activity.