Cyanobacterials (Cyanophyta) belong to phytoplancton. In normal stale concentration of cyanobacterial cells in water rangs between a few hundreds to a few thousands in I drn' of water but while blooming may be increased even to one million in I dm3. At this time water has characteristic color, depending on dominated species. Also characteristic smell is the results of the presence ofcyanobacterial and phytoplankton cells producing odour substances. The cyanobacterial blooms are very important hygienic problem for both human and animal health. While blooming they form foams and head coating on water surface. Also cyanobacterial toxins are huge problem. Cyanobacteria may produce acute toxins such as hepatotoxic peptides (microcystins, nodularins and cylindrospermopsin) and neurotoxic alkaloids (anatoxin-a, anatoxin-a(s), homoanatoxin and aphanotoxins). Cyanobacterial toxins are very dangerous substances which can intoxicate hepatocytes and the nervous system in humans and animals. In this situation it is very important to remove them effectively in water pretreatment processes. In the present paper have been presented for the first time in Poland data on removing cyanobacterial toxins from water in pretreatment process with application of chlorine dioxide and ozone on the example of Sulejów - Łódź water pipe system. In period I 998-200 I the effectiveness rnicrocystin-LR removal ranged between 74--92% while for other izoforms they were between 45-94%.