Although the fiddler crab Uca arcuata (De Haan, 1833) has been widely recorded from Kii Peninsula, central Japan to Okinawa Inland, the local populations of this species have seriously declined recently. In the Red Data Book of Japanese marine and freshwater benthic animals in 1995, this species was listed in “RARE” category, which means the species with small populations and at risk of extinction. Our recent investigations (2000-2001) recorded small populations of this species in the estuaries of the Koya and Imazu rivers (Yamaguchi Prefecture) and the Nagae River (Okayama Prefecture) in Chûgoku District. In the Koya River, the average density was 3.7 individuals per m2, and the estimated number of individuals inhabited was 65100. These numbers indicate that the population size in this locality was as large as that of the Hikawa River in the Ariake Inland Sea, Kyûshû, western Japan.