We analyzed a total of 3,784 young fiddler crabs of Uca arcuata (De Haan, 1833) with carapace widths (CW) between 1.45 and 9.95mm, collected from a site on the Ohno River in Central Kyushu, Japan. All crabs with CW<3.0mm possessed two small symmetrical chelipeds. Young male crabs with a CW between 3.0 and 6.0mm exhibited one of four stages of cheliped differentiation: 1) two small chelipeds; 2) two large chelipeds; 3) only one cheliped; and 4) asymmetrical chelipeds (one large and one small). We identified 646 male and 632 female crabs with a CW between 6.0 and 9.95mm. No statistical difference from the 1:1 ratio is evident. We conclude that the basic sex ratio for U. arcuata is unity.