DNA relatedness studies (hydroxyapatite method) were done on 112 strains of citrobacteria. By using the recommended definition of a genomospecies 11 genomospecies were identified in the genus Citrobacter. These genomospecies were separable by their biochemical profiles. Citrobacter koseri (Citrobacter diversus) and Citrobacter amalonaticus proved to be homogeneous species, as previously described. C. amalonaticus biogroup 1, as described by Farmer et al. (J. Clin. Microbiol. 21:46-76, 1985), was shown to be a separate homogeneous species, which was named Citrobacter farmeri sp. nov. The Citrobacter freundii complex was quite heterogeneous. C. freundii sensu stricto, as represented by the type strain, contained only 9 of 66 strains in this complex. The remaining 57 strains were members of seven genomospecies. Genomospecies 5, containing 21 strains, was named Citrobacter youngae sp. nov. Genomospecies 6, containing 15 strains, was named Citrobacter braakii sp. nov. Genomospecies 7 and 8, each containing six strains, were named Citrobacter werkmanii sp. nov. and Citrobacter sedlakii sp. nov., respectively. Genomospecies 9, 10, and 11, each containing three strains, were not named.