Cyanobacteria of a picoplanktonic cell size (0.2 to 2.0 µm) are common organisms of both freshwater and marine ecosystems. However, due to their small size and relatively short study history, picoplanktonic cyanobacteria, in contrast to the microplanktonic cyanobacteria, still remains a poorly studied fraction of plankton. So far, only little information on picocyanobacteria toxicity has been reported, while the number of reports concerning their presence in ecosystems is increasing. Thus, the issue of picocyanobacteria toxicity needs more researchers' attention and interest. In this report, we present information on the current knowledge concerning the picocyanobacteria toxicity, as well as their harmfulness and problems they can cause.