A fraction showing in vitro and in vivo antiviral activity was isolated from the green alga Ulva fr~asciura found in the Indian Ocean off the West coast of India. The structure of its major component was established to be N-palmitoyl-2-amino-1,3,4,5-tetrahydroxyoctadecane on the basis of spectral studies. Ulva fusciu~n, a green alga (Chlorophyta) belonging to the family Ulvuceue is of wide occurrence along the Malvan region of the West Coast of the Indian Peninsula. Most of the species of Ulvu are used as food for humans and/or animals and as fertilizer for agriculture. Medicinal value for this alga has also been reported. In the course of an Indo-US collaborative project on "Bioactive Substances from the Indian Ocean", samples of Ulvu ~usciutu collected by scientists at the National Institute of Oceanography, Goa, India, were dried in the shade, powdered and extracted with ethyl alcohol. This alcohol extract showed antiviral activity against Semeliki Forest Virus (SFV) at 20 mg/mouse/7 days by giving 50% protection. We report here structural studies on the major component of this extract.