DNA-DNA reassociation studies on 52 strains of Flavobacterium meningosepticum showed two main hybridization groups about 40-55% interrelated and comprising 4 and 48 strains respectively. The larger group could be further divided into four subgroups if differences in thermal stability of the reassociated duplexes were taken into consideration. Of a total of 20 strains isolated from cerebrospinal fluid, 18 were found in one subgroup of 28 strains, which also contained seven blood isolates. The results indicate that genetically defined subgroups within the species might differ with regard to pathogenic significance.