Seaweeds represent valuable sources of bioactive compounds with different potential applications in pharmaceutical industries. Solvent extracts of detrital leaves of seaweed, Zostera spp. found as the beachcast in Aegean Coasts of Turkey was assayed for theircytotoxic activity. Extracts of methanol, ethyl acetate, and n-hexane of detrital leaves were used to determine the cytotoxicityactivities by the brine shrimp cytotoxicity test. LC50 48 h values of the extracts of Zostera spp. toArtemia nauplii varied by the solvent used. Cytotoxic activity of the solvent extracts used in this study were in the following order: ethyl acetate> hexane> methanol. The results of this investigation elicited that the detrital leaves of Zostera spp. have the cytotoxic potential, evidencing the existence of bioactive compounds in the beach-cast seaweed.