The Japanese mantis shrimp Oratosquilla oratoria usually lives in the mud bottom. This species excavates a U-shaped tunnel with a sarge and a small opening on a tidal flat; the length of burrow is proportional to occupant size. Artificial burrows of various diameters and lengths were made of polyvinyl chloride pipes and used to study this selective habit in large aquaria. The results showed that the species had a significant preference on the basis of diameter of pipe, but not for pipe length above a lower limit. These facts and behaviour in the artificial burrow suggest that a normal life-cycle is dependent upon habitation of a burrow of compatible size. An artificial burrow of favourable size permits feeding, spawning, etc. and increases survival of mantis shrimp reared in the laboratory.