A numerical taxonomic study was made of strains belonging to the genera Bordetella, Alcaligenes, Brucella, Acinetobacter, Moraxella, Neisseria, Haemophilus, Actinobacillus, and Pasteurella, together with a few strains from allied genera. They were examined for 139 characters covering a wide range of tests and properties. The strains fell into two major groups. Group A contained Haemophilus, Actinobacillus, and Pasteurella. Group B contained eight main clusters, representing, respectively, Bordetella pertussis, Bordetella parapertussis, Bordetella bronchiseptica combined with Alcaligenes faecalis, Brucella, Acinetobacter anitratus, Acinetobacter lwoffii, and two clusters containing Moraxella and Neisseria strains. Brucella and the three clusters of Bordetella (including Alcaligenes) were more similar to one another than t o the other clusters; it is suggested that the family Brucellaceae should comprise these three genera, and that the other genera should be placed elsewhere.Several families of bacteria listed in Bergey ' s ManuaI (13) represent aggregations of genera which at best can only be described as loose associations. Instead of encompassing organisms which overall are closely similar, they have been set up t o accommodate genera with one or two prominent characters in common. Such a group is Brucellaceae, or its predecessor Parvobacteriaceae (12,63). This family is the niche for those gram-negative, rod-shaped bacteria which require rich media for growth and are responsible for many human and animal infections. Their potential pathogenicity and the alleged difficulties of working with them account largely for their neglect by taxonomists. They tend to grow poorly or t o die out on the usual media, whereas the addition of blood t o increase growth often interferes with tests.Within Brucellaceae the genus Bordetella Moreno-L6pez 1952 has been one of the least studied. Three species are generally recognized: the type species B. pertussis (the whoopingcough bacillus), B. parapertussis, and B. bronchiseptica. Although these organisms have been placed in various genera (e-g., Haemophilus), in recent years their segregation as a separate genus has become generally accepted ( 1 3, 84). They do not require X factor (haematin) or V factor (nicotinamide adenine dinucleotide) as do species of Haemophilus. The only reason that B. pertussis requires blood is to remove toxic substances from growth media; the blood can be replaced by starch or charcoal (65) or by anionic-exchange resins (45).Bordetella-species are obligate aerobes, whereas species of Haernophilus are facultative anaerobes. The guanine plus cytosine (G + C ) ratios also differ, being 67 t o 70% for Bordetella and 38 to 42% for Haemophilus (20, 36, and findings reported below).The separation of Bordetella from Alcaligenes, Acinetobacter, and Brucella is more difficult to justify, and the main motive for the present work was to investigate this question. Many workers have noted points of similarity between Bordetella bronchiseptica and A k aligenes faecali...