Structure and function of the gonopod in mud crab, Scylla serrata, were studied with electron microscope techniques from June 2000 to December 2002. The ejaculatory duct of Scylla serrata opens on the coxae or sternum of eighth segment by short muscular penis. The first gonopod consists of a basal protopodite and an elongated endopodite, in which the cuticle folded the entire length of a tube which functions as an ejaculatory canal. The second pleopod fits into this cuticular fold,while the penis fits into a slit on the lateral side at the base of the endopodite. During coitus, seminal fluid is ejected by muscular action through the penis into the ejaculatory canal of the first pleopod. The ejaculate is further pushed down the canal by the pumping and inner folds and form the lateral slot. Several types of setae are quite conspicuous on the first and second gonopod. There are also some long processes and setae on the lateral slot. Pinnate setae are present on the medial and lateral margins of the basal part of the endodite and the protopodite. The pinnate setae are inserted via a characteristic annular insertion.