The in vitro antimicrobial activity of the marine green algae Ulva lactuca was examined against gram-positive bacteria, gram-negative bacteria, and a fungus. The ethyl-ether extract of algae exhibited a broad-spectrum of antibacterial activity, but not antifungal activity against Candida albicans. In particular, the U. lactuca extract showed strong activity against the bacterium methicillin-resistant Staphylococcus aureus (MRSA). This result confirms the potential use of seaweed extracts as a source of antibacterial compounds or as a health-promoting food for aquaculture.