A collection of 426 Haemophilus strains isolated from people with infectious diseases and from the normal flora of mucous membranes in humans and various animal species was studied in an attempt to revise and improve the taxonomy of the genus Haemophilus. The examinations included the determination of a number of biochemical and physiological properties, of which several had not previously been applied to the taxonomy of haemophili. The resulting data reavealed many hitherto unrecognized characters of taxonomic significance and several of the species can now be more accurately defined. The classification presented is supported by the DNA base composition of a large number of representative strains. A diagnostic key to the different taxa is presented. Haemophilus influenzae and H. parainfluenzae have been subdivided into a number of biotypes. It is possible to demonstrate a relationship between the individual biotypes of H. influenzae and the origin of the strains assigned to them. The results indicate that H. aegyptius, H. parahaemolyticus and H. paraphrohaemolyticus do not merit specific status. Four unnamed taxa of V-factor-dependent haemophili have been recognized. The name Haemophilus segnis is proposed for one of these taxa, which consists mainly of strains isolated from the human oral cavity. It is demonstrated that the name H. Ducreyi has been used for different groups of bacteria, and that only one of these groups can legitimately be assigned to the genus Haemophilus. Haemolytic V-factor-dependent strains from swine, previously included in H. parahaemolyticus, are significantly different from strains of human origin and should be named H. pleuropneumoniae. None of the strains from swine and fowls were haemin-dependent. The relationships of these strains to the species H. suis and H. gallinarum, and to H. parasuis and H. paragallinarum are discussed. Haemophilus piscium is shown not to belong to the genus Haemophilus. The taxonomic position of H. aphrophilus is uncertain and its possible relationship to Actinobacillus actinomycetemcomitans requires further study. The positive correlation found between the ecology of the strains studied and their affiliation with the different taxa is discussed.