Physicochemical analysis of seawater at three sites along the Red Sea cost of Egypt; that is Faied, Ras Gharib and Safaga for one year revealed marked seasonal and local variation in the eutrophication status of the water, temperature and salinity. Laurancia obtusa experienced both local and temporal variation in covering percentage with a maximum dominance during summer and autumn especially at Faied. The contents of proteins, carbohydrates and lipids in L. obtusa tissuesshowed peaks during summer at all sites particularly at Faied. Likewise, the mineral content of the alga exhibited distinct seasonal and local variation. Chloroform extracts of L. obtusa were more efficient as antibacterial than as antifungal. The most sensitive bacterial species was Pseudomonas aeruginosae and the most sensitive fungal species was Fusarium spp. The L. obtusa extract harvested from Safaga was the most potent among the studied sites. The antimicrobial activity of the algal extract obtained from algae collected during autumn and winter was stronger than during summer and spring. This was in marked contrast with the levels of the primary metabolites (protein, lipids and carbohydrates) which showed maximum levels during summer.