We compared seasonal changes in population structures and surface activities in the fiddler crab Uca arcuata between temperate (Yoshinogawa, Japan) and subtropical localities (Shinko, Japan). At Yoshinogawa, male courtship behavior was observed from April to July and pair formation from May to July, whereas at Shinko such mating behavior patterns were observed over two periods (June to September and February to March). Thus, the subtropical population appears to have a longer breeding period than the temperate population. Waving occurred from June to August at Yoshinogawa and during all months (except October and November) at Shinko. Waving frequency was higher during courting periods at all localities, although lower at Shinko compared with Yoshinogawa. Maximum body size and the size at maturity were smaller at Shinko than at Yoshinogawa. Surface and underground couplings were observed at Yoshinogawa, whereas only surface coupling was observed at Shinko. These differences and similarities are discussed from the viewpoint of habitat conditions and population characteristics.