Neisseria elongata subsp. glycolytica strain 6171/75 is closely similar to the type strain of N. elongata, M2, as regards DNA base composition, fatty acid content and electrophoretic mobility of two glutamate dehydrogenases, one of which showed a reaction of identity with the corresponding enzyme from M2 in double immunodiffusion in agar. The strain showed genetic homologies with strain M2 in genetic transformation at a level suggesting species identity, and with N. meningitidis at a lower level. No affinity to Moraxella species or "false neisseriae" was demonstrated, with the exception of a production of a few transformants in the 6171/75 recipient by DNA from Kingella kingae. The strain showed the same pattern of associated variation of colony type, fimbriation and competence in transformation as that found in other Neisseria and Moraxella species. After continuous subcultivation for some time some clones of the strain appeared to have lost the ability to produce acid from glucose.