Fish embryos and larvae are frequently subject to chronic and damaging exposure of crude cyanobacterial biomass. The hatching rates (treatment with 4 eggs per 10 mL well), achieved in the majority of experimental treatments with cyanobacterial biomass, were significantly different from the control group. Also hatching onset was considerably delayed due the presence of cyanobacterial biomass. The embryotoxicological impacts were more pronounced in higher concentrations of cyanobacterial biomass (200 mg . L-1) compared to lower concentrations (40 mg . L-1). Whilst deformities were not recorded in the control, their highest incidence amounted to 11.8% and 40.9% in 40 and 200 mg . L-1 of cyanobacterial biomass, respectively.