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The ecology of the pea crab Pinnixa tumida living in the endobenthic holothurian Paracaudina chilensis, which buries its body in the fine sandy bottom in the sublittoral zone, was investigated. Almost all of the pea crabs collected from their host holothurians were mature females. Mature pea crabs were found to live singly in the alimentary canal of the larger hosts, which are mainly distributed just below the low water level of the spring tide. However, immature crabs were not found in either large or small hosts. In the laboratory, mature crabs searched for the tip of the host's tail, which appeared on the sand's surface, and then entered the host's alimentary canal through the opening of the anus at the tail's tip. These results indicate that the pea crab enters larger hosts only when it is mature, or just before maturity. Pea crabs showed 2 types of feeding preference in the laboratory: suspended particles and the mucus secreted in the alimentary canal of the host. Ovigerous females were found in the hosts between late February and mid-May, carrying eggs which developed slowly and synchronously with those of almost all of the other ovigerous females observed throughout this period, indicating that the pea crab has an annual breeding cycle. The biology of the pea crab was compared with that of crabs living in bivalves and on irregular echinoids.