The present study was performed to identify by Gas Cromatography/Mass Spectrometry (GC/MS) the main compounds in fractions two, three and four of the hexane extract of red seaweed Gelidium serrulatum based on the antibacterial activity moderate exhibited. The main compounds identified were saturated and insaturated hydrocarbons, alcohols, phenols, lactones, ketones, fatty acids and sterols. The fatty acids identified were myristic, caproic, lauric and palmitic, and the sterols colest-5-en-3β-ol, colest-4-en-3-one, 4,6-colestadien-3-one and colestan-3,6-dione. In marine algae these compounds act as antimicrobials or antifouling and probably are responsible for the biological activities displayed by G. serrulatum.