INTRODUCTION-Since its introduction to the New Jersey coast in 1988, the western Pacific coast crab Hemigrapsus sanguineus has spread to inhabit rocky intertidal locations along the Atlantic coast from Massachusetts to North Carolina (McDermott 1998). Many reasons have been proposed to explain the rapid spread of this non-native species. For example, H. sanguineus has been shown to have longer spawning periods along the mid-Atlantic coast than in its natural habitat in the western Pacific Ocean, due to a more favorable climate (Epifanio et al 1998). For this reason, these crabs are able to spawn more times per season than native crabs, providing a possible hypothesis for their population explosion. For this species to expand its range along the Atlantic coast, it will need to have broad tolerance to temperature and salinity. . In 1998 Epifanio discovered that the purpose of this study is to show the tolerance and behavioral responses of H. sanguineus to changes in water and air temperatures and water salinity concentrations. It is believed that these crabs will be very tolerant of the various extreme conditions to which they will be subjected. The key to their success is the ability of these crabs to survive in these unfavorable situations. This experiment was also designed to demonstrate the hypothesis that the tolerance of H. sanguineus to various environmental factors increases with size. METHODS AND MATERIALS - In February 2000, a field trip was made to Crane Neck Point to collect live samples for the experiment. The trip was conducted at low tide. The water temperature was around 3 degrees Celsius, with the air temperature slightly above freezing (0-1 degrees Celsius). Live crabs were obtained by tipping rocks in the intertidal zone. Hemigrapsus sanguineus was found at all levels of the intertidal zone, although their numbers increased as one moved towards the waterline. The crabs were collected regardless of size. The sizes of the collected specimens ranged from 0.5 to 4.2 cm. The crabs were collected in a five-gallon plastic bucket. Water was added to the bucket to prevent the crabs from dehydrating. The crabs were brought back to the laboratory, where they were kept in five-gallon plastic buckets for a few weeks until the experiment began. Air tubes were added to the buckets to oxygenate the water. The water was changed if necessary. The first experiment conducted was on water temperature and salinity tolerance. The goal was to conduct an experiment that would provide measurable data on the tolerance of H.