A bacterial strain collection has been established consisting of 423 strains of Neisseria meningitidis serogroup A isolated from 23 epidemics or outbreaks that have occurred since 1960 as well as from earlier epidemics and from numerous nonepidemic situations. A combination of starch gel electrophoresis of seven cytoplasmic enzymes and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of two outer membrane proteins was used to resolve the clonal population structure of these bacteria. Fifty electrophoretic types were assigned to 21 clones on the basis of a cluster analysis. The clones were separated into four distinct serogroup A subgroups, all of which were isolated from cases as recently as 1983. Most epidemics or outbreaks were characterized by their association with a single or predominant clone, although some epidemics were apparently of mixed etiology and others yielded rare isolates belonging to other clones. Seven predominant clones were recognized that have caused sets of epidemics since 1915. At least two of these sets can be considered to represent mutually exclusive pandemics first detected in 1967 and 1973, respectively. The results define a new typing scheme, which can be used for a comprehensive description of former and future epidemics. A list of strains and their epidemiologic data is appended.