Background.The present study was carried out to evaluate the anti-ulcer activity of the brown seaweed "Cystoseira sensu lato".Methods. An acute toxicity study of the aqueous ethanol extract and the powder of "Cystoseira sensu lato" on wistar rats were carried out, and the LD50 were determined. Various concentrations of the brown seaweed (50, 100, and 150 mg/kg) were used to determine the anti-gastric ulcer activities of the algae in rats in an ethanol-induced model and the ED50 was determined .Omeprazole (50 mg/kg) was used as a reference drug. Results. The data obtained showed a zero degree of toxicity with an LD50 equal to 10,000 mg/kg. A strong anti-ulcer effect was noted for C. sensu lato algal powder.The different doses of seaweed (50, 100 and 150 mg/kg) significantly reduced the percentage of ulceration with a high percentage of inhibition. It was found that "Cystoseira sensu lato" at 150 mg/kg gave a maximum effect in reducing gastric ulceration .By following the dose-effect curve, we can estimate ED50 at 75 mg/kg. Conclusion.The study showed that the brown algae "Cystoseira sensu lato" possess remarkable antiulcer activity .It would therefore be interesting to carry out further studies to confirm other therapeutic activities of this miraculous algae.