1. The social behavior patterns of twelve species of hermit crabs found in the waters around Curaçao, N.A. are described. All species showed marked similarity in their aggressive displays, the most common of which are movements of the appendages, called here the ambulatory raise and cheliped extension. 2. Model presentation experiments proved that these positions are effective visual stimuli. These tests also showed that the white tips of the ambulatories of Clibanarius tricolor are aggressive stimuli. 3. Pagurid crabs showed a dislodging-shaking behavior pattern when crawled upon by other individuals. Experiments were carried out to determine the relationship between stimulus weight and the size of a crab showing this pattern. 4. Measurement of laboratory and field distributions indicated that some species are truly gregarious (Clibanarius tricolor, Pagurus miamensis, Pagurus bonairensis), while other species are contagiously distributed due to orientation to certain physical factors in their environment (Calcinus tibicen). 5. Laboratory and field tests showed that individuals of Clibanarius tricolor form relatively stable groups. The groups are formed and/or maintained through orientation to a “grouping pheromone”. These groups are formed daily after the crabs have been dispersed over the nightly feeding area. At night, individuals of Clibanarius tricolor oriented chemically toward a detritus-covered rock, their normal food source. Groups of Pagurus miamensis also oriented chemically to a group of conspecific individuals established on a rock. Individuals of both species oriented toward a conspecific group only during the day. 6. Diel cycle measurements were carried out for most species. The most common pattern was a nocturnal, crepuscular-peaked cycle, although Paguristes species showed an anti-crepuscular pattern. 7. Individuals of all species fought one another for gastropod shells. With the possible exception of the genus Paguristes, the direct application of force did not play a part in these shell-fights. The signals exchanged by an interacting pair were very different in the two families; in the Paguridae, the attacker shakes the defending crab back and forth rapidly by a movement of its ambulatory legs while the diogenid aggressor strikes the defender’s shell with his own by means of abdominal muscles. Measurements of shell-fighting pairs of Clibanarius tricolor indicated that smaller individuals very rarely win over larger crabs, females have a slight advantage in shell-fights and that recently moulted crabs are both more likely to be attacked and more likely to lose when attacked. 8. The sexual behavior of most species was observed and described. The precopulatory acts of the male are similar within the Families; diogenid males mainly rotate the female around an axis through the plane of her shell aperture, while pagurids jerk the female toward the male by movements of one of their chelipeds which grasps a female ambulatory leg. The normal larval releasemoult-copulate sequence was observed in most species, although pairs of Pagurus bonairensis consistently copulated while the female still had a complement of well-developed eggs. 9. A preliminary investigation indicated the presence of social order in groups of Clibanarius tricolor, Calcinus tibicen and Pagurus miamensis, but the basis for this order was uninvestigated.