The midgut of adult female Anopheles aquasalis presents a narrow anterior or thoracic region and a distensible posterior or abdominal region constituted by the epithelium formed by a cell layer whose apical portion presents microvilli and the basal portion, a basal labyrinth. The thoracic region revealed heterogeneous cellular staining affinity mainly by the presence of acidic components. The ultrastructural aspect showed columnar cells with the presence of the vesicle, mitochondria, endoplasmic reticulum and secreting cells. The abdominal region of the midgut revealed an irregular epithelium whose cells presented a basophilic cytoplasm and acidophil granules. It was also found secreting and/or basal cells with narrow cytoplasm. The ultrastructural observation of this region demonstrated cells with evident nucleus, endoplasmic reticulum and mitochondria. Larger vesicles and small granules were found distributed throughout the cytoplasm. The basal lamina that supports the epithelium presented a generally irregular aspect and the muscle fibers have longitudinal and circular organization and were found separating the epithelium from the haemocel. This study will contribute to analyses on the vector mosquito-parasite interaction mechanism in this specimen.