SUMMARY: Eleven strains regarded as moraxellas were studied j nine were charac- terized by moderate growth on the usual media, a positive oxidase reaction and sensitivity to penicillin, and six of these preferred or required a humid atmosphere when incubated at 37'. The classification of these strains was considered correct. Two strains did not show any of these characteristics, and it was concluded that they should not properly be classified as moraxellas. A positive oxidase reaction, sensitivity to penicillin, and preference for a humid atmosphere at 37" may be useful criteria in the classification of Moraxella. It is suggested that Mormella may be closely related to Neisseria, and that the descrip- tion of the genus and its taxonomic position should be revised in accordance with Lwoff's proposals. Mima polymorpha var. oxidans (De Bord) is believed to belong to Moraxella, whereas other members of the tribe Mimeae (De Bord) and Bacterium anitratum (Schaub & Hauber) are probably not closely related to Moraxella. Lwoff (1939) proposed that the organism classified as Haemophilus duplex and some related organisms should be gathered in a new genus, Moraxella, and that this genus should be separated from the Haemophileae, as its species neither resembled the Haemophileae morphologically nor required haematin or phosphopyridinenucleotide as growth factors. These proposals were approved by Audureau (1940), who added a new species, M. lwofl. Bergey's Manual (1948) accepted Lwoff's proposal to create the genus Moraxella, but put it in the Haemophileae. The vague description of the genus-mainly morphological, which might fit almost any Gram-negative rod-indicates a lack of specific characters to differentiate it from other genera. The present study was made to evaluate additional characters, which, added to the description, would clarify the taxonomy of the genus and show its relationship to organisms such as Mimeae (De Bord, 1939), Bacterium anitratum (Schaub & Hauber, 1948) and Neisseria, all of which appeared to resemble Moraxella in some respects. MATERIAL AND METHODS Of the strains isolated in this laboratory, one (3452/51) was from sputum (Henriksen, 1951), three from urine (283/49, 6758/51, 7146151) and two (possibly the same strain) from throat cultures of a husband and wife (OT/51, AT/51). Type strains of some Moraxella species were received from the Institut Pasteur, Paris, through the courtesy of Dr A. Lwoff and Dr M. Pibchaud (M. lacunata 4238, Diplobacillus of Morax 260, Diplobacillus of Petit, M. lwofl var. bacteroides and M. pericardite). For comparison five strains of Neisseria pharyngis, four of the pigmented, carbohydrate-fermenting variety, and one non-pigmented, non-fermenting (N. catarrhalis), were studied (OT 2/51, OTJ 1/51, OTJ 2/51, TT/51,26051/51).