The valuation of algal biomass is considered among the most interesting in the use of the marine international programs. On the other hand, Morocco, with its double Atlantic and Mediterranean coast, is a country deeply influenced by the sea, which implies the existence of thousands of marine species. This work involves the use of extracts of brown seaweed Cystoseira tamariscifolia in various biological activities such as: antibacterial activity, antileishmanial activity and cytotoxicity activity. Briefly, samples are obtained from the Soxhlet extraction successively using solvents of increasing polarity (hexan, ether and chloroform ) , and they represent 0.22 % to about 5.67 % by weight of raw seaweed. The antibacterial activity was carried out by the method of diffusion in agar Legard Escherichia coli, Enterobacter cloacae, Klebsiella pneumoniae and Staphylococcus aureus. The antileshmanien activity was carried out in vitro by MTT colorimetric method (3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium Bromide) by Leishmania infantum, and cytotoxicity was achieved a larvae brine shrimp : Artemia salina (Brine shrimp). The overall results show that the products extracted from seaweed Cystoseira tamariscifolia exhibit remarkable biological activity.