Serotonin is an important neuromessenger used in a variety of signaling pathways throughout the animal kingdom. In insects, serotonin has been demonstrated to mediate feeding and feeding-related behaviors. In this study, serotonin antibody was localized in cells and processes throughout the central nervous system (CNS) and midgut of female horse fly Tabanus nigrovittatus Macquart. In the CNS, immunoreactivity was localized in cells and processes throughout the brain and ventral nerve cord. In the midgut, a fine network of immunoreactive processes was observed running along the outer surface of the midgut, with a decrease in innervation toward the posterior region of the midgut.