We are interested in the molecular events underlying the development of the nervous system of Manduca sexta during the final 24 h of the pupal molt. In this article we describe a gene, MnglO, that is expressed in the abdominal nervous system of M . sexia and is developmentally regulated over this 24-h period. I n situ hybridization analysis shows that the transcript is localized predominantly to a single pair of uniquely identifiable neurosecretory neurons, the NS-LI cells in the abdominal ganglia. MnglO is a single copy gene encoding a 229 amino acid protein with a predicted molecular mass of 26 kDa. At the amino acid level the protein shows 34% identity to the yeast transcription unit, Yer082. Northern blot anal-ysis shows that the transcript of MnglO is very rare, comprisingabout 0.001% of the poly (A)' RNA from the CNS and is detectable at 4 h but not 24 h prior to pupal ecdysis. One of the physiological events that develops over the final 24 h of the pupal molt is the ability of the nervous system to respond to the neuropeptide eclosion hormone. In this context, it is interesting to note that the NS-L, cells are members of the group of 50 neurons that show increased cCMP immunoreactivity when the nervous system is exposed to the neuropeptide eclosion hormone.