A numerical taxonomic study was made of strains belonging to the genera Bordetella, Alcaligenes, Brucella, Acinetobacter, Moraxella, Neisseria, Haemo- philus, 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 to the other clusters; it is suggested that the family Brucellaceae should comprise these three genera, and that the other genera should be placed elsewhere.