Phormidium (now called Microcoleus) is a cyanobacterium (toxic algae) that grows in some New Zealand rivers. It produces neurotoxins called anatoxins and ingestion of cyanobacteria mats has caused numerous dog deaths. In order to manage Phormidium in our rivers, we need more knowledge on the toxicity of these anatoxins (i.e., the amount of toxin that needs to be ingested to cause harm). Two different anatoxins produced by New Zealand Phormidium (anatoxin-a and dihydroanatoxin-a; ATX and dhATX) were purified from New Zealand cyanobacteria and assessed for their acute (short-term) toxicity to mice. The toxicity work showed that dhATX is more toxic than previously thought. The acute toxicity values generated through this work will be used for risk assessments of toxic cyanobacteria in our waterways.