Toxic cyanobacteria found in eutrophic, municipal and residential water supplies are an increasing environmental hazard in South Africa. Cyanobacteria produce lethal toxins, and domestic and wild animal deaths are caused by drinking water contaminated by these toxins. Among the species causing death of livestock, blooms of Microcystis aeruginosa
are the most common in South Africa. More than 65 microcystins have been isolated to date and they are the most abundant cyanobacterial toxins. Hazards to human health may result from chronic exposure via contaminated water supplies. Microcystins are powerful tumour promoters and inhibitors of protein phosphatase 1 and 2A and they are suspected to be involved in the promotion of primary liver cancer in humans. In this minireview, we discuss the significance of toxic cyanobacteria in South Africa as well as the detection of potential microcystin-producing cyanobacteria strains in South African reservoirs with a mcyB molecular marker. It would be of economic and public health value to be able to detect early stage blooms of cyanobacteria, especially if it is on a sufficiently timely basis for municipalities and recreation facilities to implement a response plan.