Oligomycin is an inhibitor of the mitochondrial ATP synthase. In nitrogen-replete cells of the marine diatom Phaeodactylum tricornutum Bohlin, the rate of dark respiration was high and markedly inhibited (62%–74%) in the presence of oligomycin. In contrast, the rate of dark respiration in nitrogen-deprived cells was about half that in nitrogen-replete cells but was only slightly inhibited (16%–30%) by oligomycin. Consistent with these effects on rates of dark respiration, oligomycin decreased the ATP level and the ATP:ADP ratio by about 40% in nitrogen-replete cells incubated in darkness but had a negligible effect on the ATP level and ATP:ADP ratio in nitrogen-deprived cells. In sodium and nitrogen-deprived cells, the rate of dark respiration was greater than that in nitrogen-replete cells, but there was little effect of oligomycin on the rate of dark respiration. In light-limited cells, the rate of dark respiration was similar to that in nitrogen-deprived cells, but the inhibition (57%) in the presence of oligomycin was greater. These results suggest that most of the O2 consumption by nitrogen-replete cells was linked to mitochondrial ATP synthesis and that the rate of mitochondrial ATP synthesis in nitrogen-deprived and sodium and nitrogen-deprived cells was low. The potential implications of these results for our understanding of maintenance respiration are discussed.