The gene encoding the root adhesin from the outer membrane of Pseudomonas fluorescens OE 28.3 was isolated from a genomic lambda EMBL3 library and sequenced. The deduced protein (32104 daltons) displayed strong homology with the amino- and carboxyterminal parts of porin F (OprF) from P. aeruginosa and P. syringae. Significant homology was also found within the C-terminal domain of the OmpA proteins from Enterobacteria and major outer membrane proteins from Neisseria species. However, a cysteine-rich domain present in the OprFs of P. aeruginosa and P. syringae is absent from the adhesin of P. fluorescens. Instead, it contains a shorter sequence with eight alternating proline residues.