In the mosquito Aedes aegypti, 5-HT changes the endogenous rhythm of contractions in the female hindgut and increases fluid secretion in the larval Malpighian tubule. The role of 5-HT as a diuretic hormone in adults has been questioned. We cloned a cDNA encoding a serotonin receptor from a female A. aegypti Malpighian tubule library that is similar to the 5-HT7 receptor from Drosophila melanogaster. The transcript was localized in the tracheolar cells associated with the female Malpighian tubules but no signal was detectable in the tubule epithelium. Immunohistochemistry with specific antibodies confirmed the receptor expression in tracheolar cells and hindgut, and western blots of these tissues showed the expected 50 kDa band. The results suggest a role for serotonin in respiration and that this receptor may coordinate the tubule-hindgut response to serotonin during diuresis.