Antisera against a neuropeptide, allatostatin (AST), inhibiting biosynthesis of juvenile hormone (JH) in vitro by corpora allata (CA) have been used to analyze of the number, localization and structure of allatostatin-immunoreactive (AST- IR) neurons in the central nervous system (CNS) of the silk moth, Bombyx mori, during postembryonic development. No AST- IR neuron was found in the CNS of 1st and 2nd instar larvae. AST-immunoreactivity began to appear in the brain of the 3rd instar larva, in which only brain contained two strongly labeled pairs of bilateral AST-IR cell bodies. However, weak allatostatin- immunoreactivity also occurred in the suboesophageal ganglion. Allatostatin-immunoreactivity in the brain and ventral ganglia of the 4th ins tar larva showed very similar pattern to that in the 3rd instar larva, except for presence of strong immunoreactivity in one pair of AST-IR neurons in the middle region of whole suboesophageal ganglion of the 4th instar larva. In the 5th instar larva, seven strongly labeled pairs of bilateral AST-IR neurons projected their axons into the retrocerebral complex through ipsilateral nervi corporis cardiac (NCC) II and nervi corporis allati (NCA) I. This fact suggested that allatostatin might playa major role on inhibition of JH biosynthesis by the CA. Sub oesophageal ganglia of the 5th instar larva and pupa had location of two pairs of AST-IR neurons in the middle region, respectively. The 5th instarb larva has one or two pairs of AST-IR neurons in the lateral region of abdominal ganglia (AG) 1 to 7/8.