Burrow structure of the thalassinidean mud shrimp Upogebia major (de Haan) was investigated on a tidal flat in Tokyo Bay, central Japan, using in situ resin casting. Burrow structure consisted of two sections, an upper U-shaped part and a lower I-shaped part, with chambers and short branches on each. Depth of the burrow reached 208 cm, and extrapolations from the data on two incomplete burrows indicate that the maximum depth may exceed 250 cm. The depth of the burrow increased exponentially with carapace length of the inhabiting shrimp. This increase was accounted for principally by vertical extension of the lower I-shaped section. Each burrow was occupied by only a single shrimp. Even small new recruits occupied their own independent burrows. In this respect the mud shrimp proved different from some other thalassinidean shrimps, in which juveniles attach their burrows to those of adults. Feeding and excavating habits of the mud shrimp were considered as explanations for this behavior. Numerous tubes of suspension feeding phoronid worms attached to the burrows. Burrows of the mud shrimp play an important role in oxygenating deeper sediments and structuring the local tidal flat benthic community.