The construction of a chimney after pairing is first described for Uca formosensis Rathbun, 1921, a fiddler crab endemic to Taiwan. After attracting a female into his burrow, the male builds a chimney averaging 9.2 cm high and 7.7 cm in diameter at the base within a 4- or 5-d period before the next neap tide. It is not related to sexual attraction because the chimney does not appear prior to or during courtship. To build the chimney, the male excavates the burrow which probably widens the shaft and, more importantly, deepens the burrow so that it reaches the water table, thus providing a moist chamber in which the female can incubate her eggs. The mudballs dug out from within the burrow are piled around the entrance and form the chimney, which herein is assumed to function as a way for the builder to hide from its enemies. The sex ratio of U. formosensis present on the surface was also recorded and is discussed.