We utilized microcosms that mimicked the rocky intertidal community to examine the predatory impacts of two introduced crab species, Carcinus maenas and Hemigrapsus sanguineus, on naturally occurring assemblages of organisms on rocks in the laboratory and in the field. The two crab species had similar consumption patterns causing significant declines in Semibalanus balanoides (barnacles), juvenile Mytilus edulis (mussels), Spirorbis sp. (polychaetes), and ephemeral algae. In both two-day and fourteen-day field experiments, the decline in S. balanoides in the H. sanguineus treatments was significantly greater than in the C. maenas treatments suggesting that H. sanguineus may have a higher per capita impact on barnacles than C. maenas. As H. sanguineus increases in abundance in northern New England, it may have greater impacts on prey species than C. maenas did when it became established.