To explore the locations of serotonin (5-HT) and neuropeptide Y (NPY) in the brain of Scylla serrata, immunocytochemical method of Strept Avidin-Biotin-Complex was applied to observe the immunoreaction to two antibodies. Among all the 12 somal clusters and 11 neuropils of the brain in S. serrata, 5-HT-like immunoreactivity occurs in 4 somal clusters and 6 neuropils in the protocerebrum and deutocerebrum; while NPY-like immunoreactivity is present in 7 somal dusters and 3 neuropils in the deutocerebrum and tritocerebrum. The specific distribution patterns of these two immunoreactive substances may thus provide morphological proofs for their different neurophysiological functions.