Abstract:Macroalgae from Fucus genus are a valuable source of bioactive components as they are abundantin complex polysaccharides, fatty acids and polyphenols. In this work, the biological activity and chemical composition of extracts and fractions obtained from endemic Fucus virsoides J. Agardh species collected in the summer and the fall were investigated. From dichloromethane:methanol (1 : 1) extract three fractions were made: petroleum-ether, ethyl-acetate and n-butanol. The aim of the study was to examine the influence of the seasonal variations on algal composition and activity. The significant seasonal variation in content and biolog-ical activity of Fucus virsoidessamples was found. Fall extract and fractions exerted higher cytotoxic effects on cancer cell lines in comparison with summer extract and fractions. The examined extracts and fractions showed higher cytotoxic activity towards cancer cells compared to normal fibroblast MRC-5 cells.Morphological evaluation and cell cycle distribution analysis demonstrated their pro apoptotic activity in human cervical adenocarcinoma HeLa cells. Fall extract and fractions better suppressed the migration and tube formation of EA.hy926 cells in comparison with summer extract and fractions. Fall extract and fractions were more potent in inhibition of α-glucosidase enzymatic activity. Ethyl-acetate fractions, from both seasons, exhibited the best antibacterial and antifungal activity on all tested bacteria and fungi. In conclusion, the two fall fractions ethyl-acetate and petroleum-ether rich in polyphenols and polyunsaturated fatty acids were the most active and exhibited prominent anticancer and anti-α-glucosidase activities