Brown seaweeds in the genus Dictyopteris produce several C11 sulfur metabolites that appear biosynthetically related to the C11 compounds known to attract sperm to female gametes of many brown algae. All four of the C11 sulfur compounds that we tested strongly deterred feeding by the amphipod Ampithoe longimana but had no effect on feeding by the sea urchin Arbacia punctulata, even when tested at concentrations that were two to eight times greater than those that deterred amphipods. In numerous previous investigations, a variety of seaweed compounds have been shown to deter feeding by large mobile herbivores such as fishes and urchins but to be relatively ineffective against mesograzers, such as the amphipod of our study. Our results for the C11 sulfur compounds from Dictyopteris thus contrast sharply with patterns from previous studies and suggest that these metabolites may be defenses specifically targeted against small mesograzers such as amphipods. The occurrence of C11 metabolites in brown algal eggs could allow these defenses to be especially important in defending gametes, zygotes, or young sporelings from herbivorous mesograzers.