Three collections of strains of Neisseria meningitidis that caused meningococcal disease during nonepidemic periods were serotyped to determine whether serotypes that cause endemic disease are more heterogeneous than those responsible for epidemic disease. Thirty-four strains isolated from pediatric patients in Houston, Texas, from February 1977 to March 1978 were of three separate serogroups and 11 serotypes; 27 contemporary (1977-1978) strains from predominantly military populations, obtained nationwide, were of six serogroups and six serotypes, while 11 strains isolated at military posts in the southwest United States from 1970 through 1976 were of four serogroups and five serotypes. Between 9% and 20% of the strains were nontypable, while type II strains which were responsible for the epidemics in the Northern and Western Hemispheres earlier in the 1970's, accounted for only 20%-44% of the strains. In contast to epidemics, which appear to be caused by a single serotype, endemic meningococcal disease appears to be caused by a broad, heterogeneous distribution of serotypes. Thus, development of a serotype-specific vaccine may have limited application to the prevention of endemic meningococcal disease.