The Asian shore crab Hemigrapsus sanguineus is the principal invasive crab in many intcrtidal habitats on the east coast of the United States between Cape Hatteras and Maine. Previous work has described general aspects of mating for the genus Hemigrapsus; however, there has been no explicit study of mating behavior in H. sanguineus, and there has been little consideration of how this behavior might influence the success of H. sanguineus as a bio-invader. The objective of our investigation was to document the mating behavior of H. sanguineus under laboratory conditions and compare this pattern of behavior with that of other species of Hemigrapsus. Our observations show that mating in H. sanguineus was always initiated by the male and that both the male and female had calcified exoskeletons at the time of copulation. Small males were sometimes successful in securing mates, even in the presence of larger males. However, the male was larger than the female in 67% of the observed copulations. Duration of copulation in H. sanguineus was relatively long compared with other species of Hemigrapsus, and averaged ∼30 min. H. sanguineus differs from other species of Hemigrapsus in terms of positioning during mating. H. sanguineus typically used a vertical position relative to the substratum, with both crabs standing and facing each other, ventral surface to ventral surface. This position facilitated some degree of mobility in the copulating pair. Females were receptive to copulation within a few days of larval release, and were also capable of storing sperm and producing at least 2 broods from a single copulation. Mean incubation time of newly extruded eggs was around 16 days, and females often produced a second brood within a few days of larval release. Thus, an individual H. sanguineus could produce 2 broods within 40 days and presumably additional broods with subsequent copulation. This pattern of reproduction provides enhanced fecundity relative to co-occurring native species of crabs.