Bioturbation of soldier crab, Mictyris longicarpus var. breuidactylus was studied. Its burrows have been observed all the year around with average of 8 individuals/25cm×25cm sandy tidal flat in front of Rhizophora stylosa community at the estuary of Nadara river, Iriomote Isl., Okinawa, Japan. Bioturbation by soldier crabs was estimated to be approximately 56ton dry sediment/ha/tide, affecting significantly the sediment dynamics of mangrove ecosystems.