The succinoxidase system ofAspergillus nigerdiffers from that of avian and mammalian tissues in its greater lability during preparation and storage but is similar in its sensitivity to malonate, cyanide, azide, and B.A.L., its association with cell particles, and its single pH optimum at pH 7.3. The system is not inhibited by arsenite in concentrations up to 0.01 M.