Photoautotrophic growth of the Em-R het− nif2− strain of the blue-green alga, Nostoc muscorum, is prevented completely by 1 mM methylamine (MA) but a concentration of 2 mM MA is required to inhibit photoheterotrophic or dark heterotrophic growth. From this strain a mutant has been isolated and characterized. The mutant cannot grow photoautotrophically but its heterotrophic growth is resistant to 5 mM MA; moreover, the mutant can use MA as a carbon and nitrogen source for growth. It is suggested that because the mutant is defective in photosynthetic ability it lacks the step that is inhibited by 1 mM MA and, in the absence of such inhibition, is able to show the ability to metabolize MA by an existing enzyme system. Mutant strain Em-RMSO-R het+ nif+, whose growth is resistant to inhibition by l-methionine-dl-sulphoximine, can grow photoautotrophically in 2 mM MA. Because this mutant is known to contain an altered glutamine synthetase, it is suggested that this enzyme may be responsible for methylamine metabolism.