The purpose of this research is to describe and to compare the morphology of megalopa and the first crab of four Scylla species which lives along the coast of Tungkang. The megalopa of S. serrata has dactyl lower margin on the fifth abdominal segment which is different from three other Scylla species. S. olivacea has setae number of 4,4 combination on both sides of the basal segment of antenna. It is different from three other species which are either ¡§ 3,3¡¨ or ¡§ 3,4¡¨ or ¡§ 4,3¡¨ combination. S. tranquebarica has setae number of 4,5 combination on the last two segments of antennule. It is different from three other species, which have 3,3 combination. In addition, these four kinds of megalopa are different on the fifth pleopod, third maxilliped and fifth pereiopod. Furthermore, megalopa of four Scylla species have significant differences in their shape and size. For example, S. olivacea is the largest one in both rostral width and carapace length, followed by S. serrata, S. paramamosain, and S. tranquebarica is the smallest one. In the first crab, as adult, significant differences are found on the frontal median spines and chelipeds. From September 1999 to September 2000, megalopae of four Scylla species were collected from the coast and offshore of Linpien and Fanzilum. Large amount of megalopa can be identified in to species using of one-step multiplex PCR. Results showed that S. paramamosain is the most abundant (67.7¡Ó21.5%) , followed by S. serrata (19.3¡Ó15.5%), S. olivacea (12.3¡Ó11.9%), and S. tranquebarica (0.7¡Ó1.7%) is the least one. Most of the megalopa (77.1%) molted during the night. The megalopa caught from coast took one day (p<0.001) less to become the first crab than those caught from offshore. S. paramamosain took one day more than S. serrata and S. olivacea.Salinity doesn¡¦t affect the time of molt.