The fiddler crab Uca arcuata (De Haan, 1835), which lives on intertidal mud flats, occasionally builds a cylindrical mud chimney that encircles its burrow entrance. Chimneys were more often found with burrows of females than with those of males (22% versus 10%). For both sexes, smaller individuals were more likely to construct chimneys. Chimneys were more abundant during the periods of male waving activity (May–August), but their abundance was unrelated to the semilunar tidal cycle. We experimentally demonstrated that individuals without burrows more often entered burrows without chimneys than burrows with chimneys. Because individuals compete for burrows, the function of chimney construction may be to decrease the likelihood that an individual loses its burrow to an opponent. The seasonal variation in chimney construction as well as differences in the probability of construction among various sex and size classes also support this conclusion.