During the mating season, males of the fiddler crab Uca lactea build semidomes of mud at the entrances of their burrows to which they attract females for mating in the upper intertidal zone. Related species build similar structures which either reduce aggression between neighboring males or attract sexually receptive females. Male U. lactea did not build disproportionately more semidomes as density increased, suggesting that these structures do not modulate aggression. Larger males built higher and wider semidomes as would be expected if the semidomes are a courtship signal. When the high tides were too low to cover their habitat and the sediment dried, males were unable to build new or repair existing semidomes. Towards the end of the mating season more small males built semidomes perhaps because large males prevented them from courting earlier when most females mated. We made two experimental enclosures, added males to one and males and females to the other, and monitored semidome building. Males built significantly more semidomes in the enclosure with females. Overall, our observations support the hypothesis that the semidomes of U. lactea are a sexual signal.