The population structure, female breeding cycle, male courting activity and the mating behaviour of the tropical ocypodid crab,Ilyoplax gangetica(Kemp 1919) were studied on the mud flat of Phuket Island, southern Thailand. The smallest sized female carrying eggs and male performing the waving display were 3.70 and 3.55 mm in carapace width, respectively. Both the male courting activity and the female breeding exhibited semilunar cycles. Males performing waving occurred in all the months of the year and during the periods between new or full moon and the following half moon on the monthly basis. After enticement into a waving male's burrow, the female copulated, laid eggs and released zoea after ca 12 days incubation. The pair was separated when the male left his burrow after the female laid eggs or within 6 days after pair formation.