This investigation has been performed to label leucokinin I-immunoreactive(LKIR) neurons in the ventral ganglia of the moth Spodoptera litura during postembryonic development, with an antiserum to leucokinin I (LK I). In the ventral ganglia, LK I-immunoreactivity begins to occur in the abdominal ganglia A3 to A 7 of the first ins tar larva. One to two weakly labeled pairs of bilateral LKIR cell bodies are located in the suboesophageal ganglion of fourth to sixth instar larvae and in the abdominal ganglia A1 to A 7 of second to sixth instar larvae. The abdominal ganglion Al of fourth to sixth instar larvae and A8 of sixth instar larva each contain one weakly labeled pair of median LKIR cell bodies. Two strongly labeled pairs of bilateral LKIR neurons are found in A3 to A 7 of third to sixth instar larvae. Abdominal ganglia Al to AS of prepupa, pupa and adult contain one to three weakly labeled pairs of bilateral LKIR cell bodies, whereas A2 to A 7 in the same developmental stages have strongly labeled pairs of bilateral LKIR neurons. Two strongly labeled pairs of bilateral LKIR neurons in each of the abdominal ganglia of larva, prepupa, pupa and adult send axons to the neuropil, and then each axon bifurcates into two axonal branches. These axonal branches form two bundles. From each of two pairs of neurons an axon exits through the posterior ventral nerve (N2) which runs to the transverse nerve of the posterior segment.