Microcystins from 73 samples of cyanobacteria collected in Danish fresh waters were quantified and the different toxins tentatively identified by high-performance liquid chromatography. Based on species compositions, the samples fell into four groups dominated by (1) Planktothrix agardhii. (2) Microcystis spp.. (3) Anabaena spp.. and (4) multiple genera. The toxin composition and content showed major differences among the four groups. The P. agardhii-dominated group contained [d-Asp']microcystin-RR as one of the major toxins. Most of the toxins detected did not correspond to any of seven available standards. These samples contained the highest mean toxin amount (470 μg g−1 dry weight). Samples dominated by Microcystis all contained microcystin-LR. and in addition most samples contained microcystin-YR and/or microcystin-RR. The mean total toxin amount in this group was 335 μg g−1 dry weight. The two groups of samples dominated by Anabaena and multiple genera, respectively, showed more diverse toxin compositions and mean toxin contents of 132 and 238 μg g−1 dry weight.