The population of the hermit crab Pagurus samuelis at Carmel River Beach, Carmel, California, was surveyed and the results were statistically treated to determine the distribution of the species. Groups within the population were observed to evaluate the stability of the groups. The population was found to be contagiously distributed with varying degrees of stability exhibited by the groups. The factors governing stability were the restriction imposed by the microenvironment and the amount of time during which the area containing the crabs was flooded. Laboratory and field observations of individual behavior showed the species to be of an aggressive nature. The learning capacity of P. samuelis was the subject of an experiment in which the performance of individual crabs in a simple T-maze was evaluated; the experiment showed that the crabs were capable of improving their performance with time. (Author).