ABSTRACT: The recent introduction of the European green crab Carcinus maenas and Asian shore crab Hemigrapsus sanguineus to the west and east coasts of North America, respectively, presents a unique opportunity for investigation intocompetitive dynamics among intertidal crabs. Juvenile C. maenas utilize rocks, shell, and other cover, and their arrival on the west coast places them in potential competition for these resources with an abundant native grapsid, H.oregonensis. Similarly, H. sanguineus use intertidal shelter on the east coast, thus placing C. maenas in possible competition with grapsids both as an invader on the west coast and as Œresident¹ on the east coast, having beenestablished there for more than 150 yr. Field sampling and laboratory experiments testing competition for space between C. maenas and Hemigrapsus spp. of equal carapace width were conducted on both coasts, as were videotaped feeding trialsto examine and quantify agonistic interactions between species. When competing for food (a single damaged, anchored bivalve), C. maenas dominated over H. oregonensis, while H. sanguineus were overwhelmingly dominant over C.maenas. Within-quadrat, stratified sampling of rocks and sand revealed striking differences in habitat utilization by C. maenas living in the presence or absence of Hemigrapsus. Only ~20% of juvenile C. maenas occurred under rocksin areas occupied by either Hemigrapsus species, while north of the present distribution of H. sanguineus (in Maine) >97% of the C. maenas were found beneath rocks. This pattern was reflected in laboratory trials as well, whereboth species of Hemigrapsus consistently dominated in contests for shelter. Given the importance of intertidal cover for small crabs, such competitive interactions will likely affect patterns of habitat use by C. maenas on the east coast andmay have important implications for the ultimate distribution and impact of this species in the northeastern Pacific.