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 to know whether the anatoxins contained inside Phormidium cells are released into river water, and if so, how much is released? Water samples and time-integrated samples (using a sampler that passively absorbs the toxins from the water) were collected over a 24-hour period at three New Zealand rivers experiencing toxic Phormidium blooms. The results of this study showed that anatoxins were consistently released into the river water and that the concentration of anatoxin in the water was related to the severity of the Phormidium bloom (a combination of Phormidium biomass in the river and the toxin concentrations in the biomass). These results will be used when developing risk assessments of toxic cyanobacteria in our waterways.