The effciency of denitrification, or anaerobic respiration, in Pseudomonas denitrificans was investigated, using growth yield as an index. Glutamate was mainly used as the sole source of energy and carbon. In batch culture, the growth yield per mole of electrons transported through the respiratory system under denitrifying conditions was about half that under aerobic conditions. Similar figures were also obtained in chemostat cultures under glutamate-limited conditions. The decrease in growth yield under denitrifying conditions could be due to the restriction of phosphorylation associated with nitrate reduction to nitrogen gas.