SubtypingNeisseria meningitidisby methods that rely on monoclonal antibody (mAb) reactivity results in an unusually high number of strains that are not subtypeable. To subtype 48 strains isolated (1993-1994) in the province of Quebec that were not subtypeable by mAb-based techniques, we used DNA sequencing of the variable regions ofporA,a gene that encodes the class 1 outer membrane protein. We assigned subtypes to all the previously nonserosubtypeable isolates and identified some novel subtypes. Because our sequencing strategy included the promoter region ofporA,different isolates were compared in their sequences of theporApromoter region. A poly(G) stretch lies between the -10 and -35 regions of the promoter; replacement of a G residue by an A residue in this region resulted in loss of expression ofporA. No correlation was found between the number of G residues in the poly(G) stretch and the level of expression; a minimum of 10 G residues is required in this stretch for expression ofporA. One isolate expressed no class 1 outer membrane protein because of the insertion sequence IS1301in the coding region ofporA. Another isolate did not express the protein owing to a frame-shift mutation within the coding region ofporA. Sequencing ofporAallowed assignments of subtypes to previously uncharacterized isolates and provided insights about the regulation of expression of this gene inN. meningitidis.Key words:Neisseria meningitidis, outer membrane proteins, subtyping, PorA, DNA sequencing.