The catabolism of glutamate and fumarate was studied by radiorespirometry in selectedNeisseriaspecies. The tricarboxylic acid cycle is functioning in all species tested, in spite of the known absence ofin vitromalate dehydrogenase activity inN. meningitidis, N. gonorrhoeaeandN. cinerea.The results imply a pyridine nucleotide independent oxidation of malate. The oxidation of glutamate is less complete in the presence of phosphate. InN. meningitidis, N. perflava, N. flava, N. subflavaandN. lactamicathe catabolism of fumarate was slow and incomplete in the absence of glutamate.