<jats:title>Abstract</jats:title> <jats:p>A neurotoxic cyanophyte (Oscillatoriales) was characterized biologically and chemically with respect to toxic properties. The organism, identified as Oscillatoria formosa (Phormidium formosum [Bory ex Gom.] comb.n. Anagnostidis et Komárek), was isolated and brought to pure culture. The dried biomass used in these investigations was grown from the strain NIVA-CYA 92 in semicontinuous culture, giving yields of about 400 mg/d dry weight of the organism, and the toxic component was extracted and separated on C18 Bond Elut® cartridges followed by HPLC purification. The purified toxin was used for structure determination by low-resolution MS and nuclear magnetic resonance. The toxin is a secondary amine alkaloid identified as methylene-anatoxin-a and was therefore named homoanatoxin-a. It is a potent neuromuscular blocking agent (LD50 [i.p., mouse] 250 μg/kg). Toxicosis in the lethal dose range leads to severe body paralysis, convulsions, and death by respiratory arrest in 7 to 12 min. Experiments with rat phrenic nerve-hemidiaphragm preparations demonstrated that the physiological effects of homoanatoxin-a are related to those observed for d-tubocurarine.