Hemigrapsus sanguineus (Asian shore crab) first arrived at Rye, NY in 1994. The intertidal abundances of H. sanguineus, Carcinus maenas (green crab), and the native crabs Eurypanopeus depressus (flatback mud crab), Cancer irroratus (Atlantic rock crab), and Libinia emarginata (spider crab) were censused from 1998–2005. Asian shore crab densities (estimated in June) increased from 1998–2001 to ca. 120 crabs m⁻², and then declined to 80 crabs m⁻² from 2002–2005. The flatback mud crab declined in abundance by about 95%. Decreases in the abundances of Atlantic rock crabs, green crabs, and spider crabsmay also have occurred, though these species were uncommon at the outset of the study. The lower intertidal density of the gastropod Littorina littorea (common periwinkle) decreased by about 80%, and the decline was coincident with the expansion of the Asian shore crab population. In June, small Asian shore crabs were disproportionately more abundant in the upper intertidal zone compared with lower zones, where large crabs were more abundant. January intertidal populations were dominated by small Asian shore crabs, and these were restricted to the lower half of the intertidal zone.