Subtyping Neisseria meningitidis by 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 of porA, 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 of porA, different isolates were compared in their sequences of the porA promoter 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 of porA. 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 of porA. One isolate expressed no class 1 outer membrane protein because of the insertion sequence IS1301 in the coding region of porA. Another isolate did not express the protein owing to a frame-shift mutation within the coding region of porA. Sequencing of porA allowed assignments of subtypes to previously uncharacterized isolates and provided insights about the regulation of expression of this gene in N. meningitidis.