The numerical, locational and structural changes of the neurons reacting with an antiserum raised against a neuropeptide, locustatachykinin-I (Lom TK I) were investigated in the ventral ganglia of the common cutworm, Spodoptera litura, during postembryonic development. In the first instar larva, one pair of the Lom TK I- immunoreactive (Lom TK I-IR) neurons are included only in the mesothoracic ganglion of the ventral nerve cord. In the second instar larva, suboesophageal, prothoracic, mesothoracic, metathoracic, and sixth abdominal ganglia contain about 2 to 14 Lom TK I- IR neurons, while all the ganglia of the ventral nerve cord in the third instar larva have increased number of the Lom TK I- IR neurons. Each ganglion of the fourth, fifth and sixth instar larvae includes the largest number of the Lom TK I- IR neurons. Thereafter, however, smaller number of the Lom TK I- IR neurons are included in each ganglion than in late larval stages. Most of the Lom TK I- IR neurons show bilateral localization in each ganglion during postembryonic development, whereas extremly small number of the Lom TK I-IR neurons are unpaired, median neurons in all metathoracic ganglia from the second instar larva to the adult, and the fifth abdominal ganglia of the 3- day- old pupa to the adult. Most of the Lom TK I-IR neurons sre the interneurons, while part of them are afferent or efferent. The extrinsic Lom TK I- IR fibers form two pairs of longitudinal fiber tracts in the ventral nerve cord, especially in each ventral ganglion. The Lom TK I- IR fibers projected from the neurons in various ventral ganglia form the commissural structures between neuropil both sides.