The ET-15 clone within the electrophoretic type (ET)-37 complex of Neisseria meningitidis was first detected in Canada in 1986 and has since been associated with outbreaks of meningococcal disease in many parts of the world. While the majority of the strains of the ET-37 complex are serosubtype P1.5,2, serosubtype determination of ET-15 strains may often be incomplete, with either only one or none of the two variable regions (VRs) of the serosubtype PorA outer membrane protein reacting with monoclonal antibodies. DNA sequence analysis of the porA gene from ET-15 strains with one or both unidentified serosubtype determinants was undertaken to identify the genetic basis of the lack of reaction with the monoclonal antibodies. Fourteen different porA alleles were identified among 38 ET-15 strains from various geographic origins. The sequences corresponding to subtypes P1.5a,10d, P1.5,2, P1.5,10d, P1.5a,10k, and P1.5a,10a were identified in 18, 11, 2, 2, and 1 isolate, respectively. Of the remaining four strains, which all were nonserosubtypeable, two had a stop codon within the VR1 and the VR2, respectively, while in the other two the porA gene was interrupted by the insertion element, IS1301. Of the strains with P1.5,2 sequence, one had a stop codon between the VR1 and VR2, one had a four-amino-acid deletion outside the VR2, and another showed no expression of PorA on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results reveal that numerous genetic events have occurred in the porA gene of the ET-15 clone in the short time of its epidemic spread. The magnitude of microevolutionary mechanisms available in meningococci and the remarkable genetic flexibility of these bacteria need to be considered in relation to PorA vaccine development.