Experimental field studies at two sites in Long Island Sound have demonstrated that the nonindigenous Asian shore crab, Hemigrapsus sanguineus deHaan 1853, is a highly mobile grapsid crab that shows limited fidelity to a particular shelter or feeding site. Recovery rates of tagged crabs differed significantly at the two sites, but no differences in site fidelity were measurable between males and females at either site. Between-site differences in percent crabs recovered may be due to differences in food and shelter availability at the two sites. There is some suggestion that familiarity with a shelter site may influence site fidelity by leading to reduced mobility. A conservative estimate (based on recovered crabs only) of the mean distance traveled in 24 hours (n = 38) was 7.43 ± 1.54 m; among those crabs recovered a distance > 5 m from the release point (n = 15) the mean distance traveled was 16.87 ± 2.23 m. The rapid, widespread dispersal characteristic of the Asian crab invasion along the east coast of the United States may be due in part to the high adult mobility and low site fidelity exhibited by H. sanguineus.