Studies with 15N demonstrated that the phototrophic bacterium Rhodopseudomonas sphaeroides forma sp. denitrificans strain IL106 cannot assimilate NO-3 but rather denitrifies it to N2. This strain also fixed N2 into cell protein, although nitrogenase activity was partially inhibited in the presence of NO-3. Strain IL106 did not assimilate NO-3, but growing cultures and washed cell suspensions incorporated the tracer from 15NO-3 via denitrification to 15N2 and then via nitrogenase into cell nitrogen. This incorporation was inhibited in cells supplied with NH4+ or in the absence of light, thus confirming the participation of nitrogenase in the assimilation of nitrogen from nitrate. This represents a novel type of N2 recycling in a photodiazotrophic denitrifying bacterium.