Nitrobacter agilis, grown through seven transfers heterotrophically in the absence of nitrite, was examined in the electron microscope. The ultrastructure of such cells closely resembled that of autotrophically grown N. agilis. It was thus futher established that the organisms growing heterotrophically were indeed N. agilis and, therefore, that N. agilis is a facultative autotroph. Acetate incorporation into poly-beta-hydroxybutyrate was confirmed cytologically.