Although native consumers are potentially important for mediating invasion by non-native species, there exist few detailed studies of interactions between invaders and native consumers. In Washington State the Japanese seaweed Sargassum muticum has become widespread in nearshore marine habitats and is known to compete with native algae. We studied the response of the native herbivorous gastropod Lacuna vincta to Sargassum muticum in the San Juan Islands, WA. Field surveys revealed that L. vincta is more abundant on S. muticum than on native kelp species. In some instances we recorded more than 1000 snails on a single individual of S. muticum. Feeding trials indicate that S. muticum is preferred over the two most common native kelp species (Saccharina subsimplex and Agarum fimbriatum). We measured the aggregational (numerical) response of L. vincta to S. muticum by quantifying the abundance of L. vincta on S. muticum across a range of S. muticum patches that varied in density. At low patch density snail abundance was highly variable, with some patches having high snail densities. In contrast, dense patches of S. muticum never contained high abundances of L. vincta. Our results suggest that the effect of L. vincta on S. muticum is mediated by food preferences and host density. Moreover, it appears that L. vincta may be most effective at reducing the performance of S. muticum in early invasion stages.