In the photosynthetic bacterium Rhodopseudomonas capsulata, NH4+ switch-off of nitrogenase activity can be mimicked by its analog, methylamine. Like NH4+, methylamine appeared to require processing by glutamine synthetase (GS) before it was effective; gamma-glutamylmethylamide was shown to be the product of this reaction. Evidence that this glutamine analog functioned directly to initiate nitrogenase inactivation was suggested first by the fact that it was a poor substrate for glutamate synthase (i.e., it was not further metabolized by this pathway) and secondly, azaserine which blocks the transfer of the glutamine amide group had no effect on CH3NH3+ (or NH4+) switch-off. These observations are taken as preliminary evidence to suggest that when NH4+ inhibits nitrogenase activity, inactivation is initiated by glutamine itself, and not a molecule derived from it. Finally, evidence was presented that R. capsulata would use CH3NH3+ as a nitrogen substrate, but lag periods and generation times increased with subsequent passages.