A new toxin produced by a Limnothrix was discovered while screening cyanobacterial samples for the presence of known and unknown toxins as part of an Australian Coal Association Research Project. It is the first time that a toxin-producing freshwater Limnothrix has been described. The features of this cyanobacterium in both field and pure cultures were consistent with the cyanobacterial species Geitlerinema unigranulatum as well as with the earlier Australian descriptions of Limnothrix cf. planctonica. However, genetic analyses closely group this cyanobacterium with the commonly occurring temperate species Limnothrix redekei. Limnothrix may occur as solitary planktonic trichomes, mats on the bottom or within the water column, or in balls floating on the surface. A fine mucilage, often confused with a sheath, may cover older trichomes. The long, thin trichomes may glide into coils, may flex and are not attenuated at the ends. The cylindrical cells are 1.6-2.0 μm wide and 5.0-6.5 μm long. Constriction at the cross walls is generally not observed and cross walls are often indistinct. Refractile granules are present in the cells - near the cell walls and sometimes distributed within the cells. Limnothrix often co-dominates with Cylindrospermopsis raciborskii in both temperate and tropical environments. In the Fitzroy River system, it is often found just above the level of the thermocline. However, toxin producing material has also been isolated from water sampled from major pipelines within the Central Queensland region.