During a 6-yr period, several thousand intertidal snails, Thais lamellosa and T. emarginata, were given individually numbered tags and times of death were noted. These snails provided most of the shells used by intertidal hermit crabs (primarily Pagurus granosimanus). Changes in the hermit crab population can be related to shell availability. The crab population increased greatly during 1969, and then declined steadily until the end of the study (1973). Changes in the crab population were highly correlated with shell availability. This hermit crab population will remain constant in size if 0.8 shells become available for each crab each month.