Rapid growth in Madagascar’s mud crab (Scylla serrata) export market between 2012 and 2014 has raised concern over the long-term sustainability of the fishery. To assess the biological adequacy of national size limits to protect spawning populations of mud crabs, the size of crabs at sexual maturity was examined in Belo-sur-Mer, southwest Madagascar. The minimum size at maturity and L50 (carapace length at which 50% of the population has reached sexual maturity) was estimated for crabs collected from seven different villages by dissection and visual appraisal of testes in male crabs (n = 300) and ovaries in females (n = 300). Ovaries showed a pronounced macroscopic differentiation in size and coloration with maturation and were classified according to six ovarian developmental stages: immature, rudimentary, developing, developed, advanced and spent. Male gonad maturity was clearly distinguishable into immature classes with thin vas deferens with few or no spermatophores and mature classes with thick white vas deferens. Analysis showed that carapace length was significantly related to maturity (p < 0.001) and minimum size at sexual maturity was different between sexes (p< 0.001) p