One hundred forty-three strains isolated from cases of diseases due to Group C meningococci in Army recruits and in civilians between 1964 and 1970 have been serotyped by means of a bactericidal assay. Six serotypes were identified. The predominance of serotype 2 strains during epidemics at Fort Dix, New Jersey (42 of 47 strains) and Fort Lewis, Washington (15 of 23 strains) and the increasing prevalence of serotype 2 in cases both at other recruit training centers and among civilians, strongly suggest the existence of an epidemic strain of Group C meningococcus. Ninety-four percent of serotype 2 strains were reistant to sulfadiazine; in contrast, only 9% of serotype 1 and 52% of serotype 4 were resistant. The serotype and the degree of susceptibility of sulfadiazine serve as two markers identifying an epidemic strain of Neisseria meningitidis.