I investigated mass wandering by the fiddler crab Uca perplexa on Okinawa Island, Japan. Many larger males wandered during the daytime low tide for four continuous days in their reproductive season. Just after exposure of their burrows they moved uphill, then returned to the burrow area before dead low tide without feeding. During the wandering phase, the males did not wave to females to initiate mating. Males resumed waving after the wandering phase. I discuss why the large males wander en masse in relation to the crabs' mating system and larval release.