Toxic compounds produced by cyanobacteria (blue-green algae)are of concern to water authorities worldwide. Algal toxins (cyanotoxins) have been identified in the United Kingdom and several other European countries as well as in the United States. Ozonation is very effective for the destruction of the important hepatotoxin, microcystin LR. Several studies have shown that toxin removal is strongly dependent on the dose of ozone, and that the effectiveness of ozonation will depend on the water quality. Anatoxin-a, a neurotoxin, is also destroyed by ozone, however, the doses required for complete destruction appear to be higher than those required for microcystins. The paralytic shellfish poisons or saxitoxins are associated with neurotoxic blooms of cyanobacteria in Australia and have also been reported in the US and Denmark. In this study, four waters, sampled after treatment and prior to chlorination, were characterized using established techniques. The waters were then spiked with algal toxins and a range of ozone doses were applied. The aim of the work was to clarify the effect of water quality parameters on the destruction of two microcystin toxins, microcystins LR and LA, anatoxin-a, and saxitoxins and to relate the results to a wider range of waters. Includes 17 references, tables, figures.