We investigated inter-male competition for female mates and intersexual interactions in underground mating (UM) of the fiddler crab Uca paradussumieri . Males search for and then enter the burrows of females that are ready to ovulate ('pre-ovigerous'). In order to ensure their paternity, these males guard the female until she ovulates the following day. Thereafter the male leaves. Intruding male conspecifics attempt to reach the female. Guarding males either fight with them (N = 27), or use the flat-claw defence (N = 96) in which the male stands in the burrow shaft and blocks the entrance with his enlarged claw. The flat-claw was a very successful defence tactic (93% success), even when the intruder was larger than the guarding male. Pre-ovigerous females accepted the first male to enter her burrow, suggesting that female mate choice does not occur. Though males that succeeded to enter the burrow of pre-ovigerous female were larger than males that failed to do so, males that succeeded UM were not larger than males that failed UM. Males that succeeded UM by a take-over were not larger than either the males that were defeated or the males that succeeded in UM after their first entering. Early localization of pre-ovigerous females was important in male mating success, as was a male's ability to defend the female before she ovulated. However, some females that were not pre-ovigerous were guarded forcibly for 2 days by males that had failed to pair with a pre-ovigerous female that day. Prolonged guarding was less successful for males than guarding for one day, probably because the males had to fight with more intruders. In addition, prolonged guarding may not be adaptive for females because they lose feeding time and mate with males that lack competitive abilities.