The presented paper could be considered as the first attempt to determine the toxicity of potentially toxic cyanobacteria in Latvian water bodies used as sources for drinking water, for fisheries and recreation. Occurrence of potentially toxic species, their toxicity and toxin content of 37 bloom samples were examined in the southern part of the Gulf of Riga (Eastern Baltic Sea), in the River Daugava and in two eutrophic lakes (Lielais Baltezers and Mazais Baltezers; their canals and infiltration basins). The River Daugava and the Lake Mazais Baltezers play a significant role in drinking water supply of Riga, the capital of Latvia. The toxicity of Nodularia spumigena blooms in the Gulf of Riga was compared with toxicity detected in the Open Baltic Sea. Sixteen strains of potentially toxic cyanobacteria (Microcystis aeruginosa, M. wesenbergii, Nodularia spumigena) representing 6 collection sites were also isolated from the Gulf of Riga and toxicity under controlled laboratory conditions was investigated. Toxicological studies were carried out by applying bioassay experiments with Artemia salina as well as by PP1 assays and HPLC. Toxin analyses were performed at the University of Dundee and Fridrich - Schiller - University of Jena.Ecological and toxicological examination of summer blooms in Latvian waters revealed the abundance of toxic cyanobacteria. Studies of occurrence and toxicity of harmful phytoplankton showed that in Latvian inland waters (in eutrophic lakes Mazais Baltezers and Lielais Baltezers and in the river Daugava) and in the coastal zone of the Gulf of Riga the main attention have to be paid to the bloom events of Microcystis spp., whereas in the Open part of the Gulf of Riga - to the bloom of Nodularia spumigena. Microcystins were detected in 92 % of freshwater samples and nodularin - in 43 % of the Gulf of Riga samples. All cultured strains isolated from the Gulf of Riga contained toxins