This paper deals with the histological structure of the pseudobranch of mosquito fish (Gambusia holbrooki), which lives in an Iraqi environment. The laboratory placed the adult fish, both male and female, in a fishbowl. The head is fixed in solution Bouin′s aqueous; using the routine method that is colored with H&E stains. The microscopic slide observations show the pseudobranch in the adult is located in the cranial region on both sides of the head, It is a submerged type. The fish features some oval parts that are connected to each other by a thick connective tissue, forming a triangular cavity in the middle, and also features an irregular dashed line of melanophore. It bears resemblance to the respiratory gills, but differs from them by losing the gill arches and their appendices, as well as by the arrangement of their components and function. This difference explains their connection to the ocular placenta and its oxygenated blood supply.