The occurrence of toxic freshwater blooms of cyanobacteria has been frequently reported during the last 15 years in Lalla Takerkoust lake (35 km southwestern of Marrakech city) In this study, it has been confirmed that the collected cyanobacteria bloom could produce different variants of microcystins at high concentration of 11.5 mg equiv. MC-LR g-1DW of the cyanobacteria cells. In order to study the effect of microcystins on faba bean seedling cultured in vitro, the crude aqueous extract of the toxic bloom was prepared and sterilized by filtration, and then it was supplemented to BNM medium at different concentrations. After 10 days of in vitro seedlings growing in BNM medium, plants fresh and dry weights were determined, plants shoot and root length was measured and then the roots were subjected to histological microscope observation of root hair, root tip and root cortical cells. The results revealed that microcystins exposure induced a decreasing of seedling growth and biomass accumulation in a concentration dependent manner. In addition, seedling young roots exhibited a brownish aspect, necrosis and tissue lysis. At a microcystin's concentration of 40