A numerical phenotypic taxonomic study of 315 strains of Neisseria and some allied bacteria examined for 155 phenotypic tests showed 31 groups, most of which were reasonably distinct. These fell into four major areas. Areas A, B and C contained species of Neisseria, whereas area D contained the organisms known as 'false neisserias' together with Branhamella, Moraxella and Kingella species. Area A contained N. gonorrhoeae (which showed two subgroups), N. meningitidis (with two subgroups, and N. cinerea closely associated), N. polysaccharea, N. elongata subsp, glycolytica and N. lactamica. Area B contained mainly organisms from the human nasopharynx, and the nine groups were not very distinct: only three, N. mucosa, N. perflava and N. sicca could be recognized by the presence of type strains, and there was little relationship between taxonomic position and species epithets. Area C contained several groups from animals, N. animalis, N. canis and two phenons that may be justified as new species of Neisseria, one from lizards and the other from dental plaque of herbivores. Area C also contained N. elongata, N. subflava (with N. flavescens), type strain of Morococcus cerebrosis and the CDC groups M-5 (N. weaveri) and EF-4. Area D contained Branhamella catarrhalis, a combined group which consists of strains of the 'false neisserias' N. caviae and N. cuniculi, the 'false neisseria' N. ovis, and a group of Moraxella strains. A small group representing Kingella kingae is included in area D. Mean test error was 1.7%.