C-2, C-4, C-5 and C-6 of the potent anticholinesterase, anatoxin-a(s), in Anabaena flos-aquae NRC 525-17 are derived from the guanido carbon, C-5, C-4 and C-3 of L-arginine, respectively, and the three O- and N-methyl groups originate from the tetrahydrofolate Cl pool. Erythro-4 Hydroxy-L-arginine, a minor constituent of the cyanophyte, may be a precursor in the biosynthesis.