Cyanobacterial blooms have been occasionally observed in Iznik lake, a freshwater body (salinity = 0.5) located in the western part of Turkey. Nodularia spumigena (Mertens in Juergens) was recorded in the lake in the summer months of 2005. Maximum filament concentration of the species (1.3 x 10(5) fil L(-1)) was measured in August and constituted 60% of total cyanobacteria abundance. Trichomes were solitary, straight and had cells containing gas vesicles. Heterocysts were regularly spaced throughout the filaments. In the isolated filaments nodularin was detected by HPLC, ELISA and PPIA as well as LC-MS. HPLC analysis showed that gravimetric nodularin concentration in cultured N. spumigena cells was 578 p,g of nodularin per gram dry weight (d.w.). Apart from nodularin, demethylated nodularin variant was also found in Nodularia cell extract. This is the first report of toxic N. spumigena in a European freshwater lake. (C) 2008 Elsevier B.V. All rights reserved.