Five bacterial isolates were recovered from various diseased birds (chickens, a pigeon, and a zebra finch) and were identified as Flavobacterium meningosepticum. Four of them were isolated in pure or nearly pure culture of samples from internal organs, and one strain was isolated in mixed culture of a tarsal joint fluid sample. Except for the last case, there was no evidence of other disease agents. By using phenotypic, chemotaxonomic, and genomic methods, the strains were taxonomically characterized and could not be differentiated from the human clinical reference strains of the species. Two avian strains were different in their phenotypic behaviors and constituted another genotypic subgroup. In general, all F. meningosepticum strains constituted a single species which was easily differentiated from biochemically similar species and phylogenetically closely related taxa.