The changes in number, localization and structure of specific neurons, using an antiserum raised against a neuropeptide, leucokinin I, were studied in the central nervous system (CNS) from the wax moth Galleria mellonella during postembryonic development. In the brains of the third, fifth and seventh larval stages, about 12 to 14 leucokinin-like immunoreactive (LK-LI) neurons were found in the protocerebrum, but no immunoreactivity was detected in the first instar larva. During the metamorphic changes from prepupa to adult, number of LK-LI neuronsdecreased and remained with the two. The LK-LI neurons generally showed complex structural characteristics that their axons become longer and more bifurcate in the brain during metamorphic courses of larva to adult, in spite of decrease in number of these neurons. In ventral ganglia, there were two pairs of large, highly prominent LK-LI neurons in each of abdominal ganglia (AG) 3-7 located as bilateral pairs during most larval stages except the first instar larval stage. The axons of each ganglion were projected into the sternal nerve (N2). Interestingly, there was a specific appearance of the ventral, unpaired, median (VUM) LK-LI cells in the AG 5,6 (for fifth and seventh instar larvae) and 7 (only for fifth instar larva) and a caudal pair of large, median LK-LI cells (MC) in the AG 7, respectively. By the pupal stages, these kinds of cells disappeared. The changes in number, localization and structure of the LK-LI neurons in brains and ventral ganglia during postembryonic development are regarded as a differentiation progress of the LK-LI neurons toward an imaginal type. The distribution of immunoreactivity in the CNS also suggests that LK-like neuropeptide is, perhaps, used as a neurotransmitter or neuromodu-later as well as neurohormone and may have an important role in signal transfer or regulation thereof.