The sequence and tissue expression of the gene encoding a peptide hormone Aea-HP-I, known to inhibit host-seeking behavior, has been characterized for the yellowfever mosquito, Aedes aegypti (L.). The open reading frame reveals a prepropeptide that would be processed into three identical peptides. The gene contains four short introns and exists as a single genomic copy. Transcripts of the gene were present in the brain, terminal ganglion, and midgut of adults, and in females, its expression profile differed for each tissue before and during a reproductive cycle. Peptides resulting from this expression were identified in the female tissues by immunoassays. Numerous neurosecretory cells and neurons in the nervous system were immunostained by an Aea-HP-I antiserum. Hundreds of endocrine cells were stained similarly in the midgut, thus contributing to the 10 times greater amount of immunoreactive peptide in an abdomen than in a head, as determined with an Aea-HP-I radioimmunoassay. Based on these results, neurosecretory cells and midgut endocrine cells are likely sources of Aea-HPs shown to reach highest hemolymph titer at the same time as host seeking is inhibited in female Ae. aegypti during a reproductive cycle.