In the female mosquito, Aedes aegypti, neurohormones are released from the brain in response to a blood meal and stimulate the ovaries to secrete ecdysteroid hormones, which modulate yolk protein synthesis in the fat body. Neuropeptides with this bioactivity were isolated from head extracts, and partial sequences from these peptides when aligned gave a 31-residue sequence at the amino terminus. Oligonucleotide primers for this sequence were used to amplify with the polymerase chain reaction a genomic DNA product that hybridized to a clone from a head cDNA library. The cDNA encodes a 149-residue preprohormone that is processed into an 86-residue peptide, as indicated by the mass value obtained from the native peptide, with the expected amino-terminal sequence. After modification, the cDNA for the putative neurohormone was expressed in a bacterial system, and the purified peptide had high specific activity in bioassays, thus confirming that it is a steroidogenic gonadotropin, the first to be identified for invertebrates.