Abstract European fiddler crabs place mudballs around their burrow openings. Both males and females placed mudballs, but there were major differences between the sexes in mudballing behaviour, suggesting that the female's mudballs were a by-product of digging out the burrow whereas the male's may have additional functions. When the male's mudballs were removed experimentally, the number and intensity of male–male agonistic interactions increased significantly. Experimentally visually isolated males spent longer making mudballs and less time waving. In a binary choice test, females were more likely to approach dummy males with mudballs, spent longer near these males and were more likely to enter their burrows than dummy males without mudballs. The same pattern was apparent for males with 30 rather than 20 mudballs. These results are consistent with a dual function for mudballs in U. tangeri : to reduce the number and intensity of aggressive interactions between neighbouring males and to attract females.