Cyanobacteria (blue-green algae) implicated in the deaths of 9 dogs at Richmond Lake, SD, on Aug 26, 1985, were analyzed. The dominant cyanobacterial species from the water sample was Anabaena flos-aquae. The lyophilized bloom material or the high-performance liquid chromatography purified toxin peak, when administered to mice IP, induced clinical signs of salivation, lacrimation, urinary incontinence, defecation, convulsion, fasciculation, and respiratory arrest. Further comparison of the semipurified bloom toxin with an irreversible anticholinesterase anatoxin-a(s), produced by A flos-aquae strain NRC-525-17, revealed the bloom toxin and anatoxin-a(s) had similar properties on high-performance liquid chromatography and on the inhibition of electric eel acetylcholinesterase (EC 3.1.1.7).