Eutrophication in the aquatic environment has been increasing because of the changing environmental condition due to natural or anthropogenic reasons, and with it comes harmful algal blooms which cause various problems within the aquatic community, especially for aquatic organisms. One of the effects of this phenomenon is the release of different toxic metabolites, one of which is Cylindrospermopsin (CYN) which is produced by some genera of cyanobacteria. The aim of the study was to study the effect of cylindropermopsin on the life cycle and morphology of the model organism Haematococcus pluvialis, an eukaryotic green alga. The results of the study have shown that CYN influences both the viability of vegetative cells and the cyst formation the green alga. According to the results of the study, a relatively high concentration of CYN (in this case 4.0 µg/ml) along with prolonged exposure (in the study 11 days period) can induce cyst formation in the model organism. However, the lower proportion of green-red cysts (GRCs) in the CYN-treated cultures than the control suggests that despite the nutrient deprivation that occurred during culture, it didn’t “overwrite” the possible effect of CYN to the alga, suggesting an inhibition in the maturation process due to the presence of CYN.