Abstract: Distribution and ultrastructural changes in the mitochondrion-rich cells in gills of artificial selected Trachinotus ovatus under different salinities (5, 20, and 30) were examined by light and transmission electron micrograph. Results indicated that the mitochondrion-rich cells were mainly present on the base of the gill filaments and branchial leaflets, and the volume and quantity of mitochondrion-rich cells increased with salinity. All three salinity groups had apical crypts, which were constituted by the mitochondrion-rich cells, pavement cells and accessory cells. Mitochondrion-rich cells in the salinity 5 group had large apical membranes with developed microridges and shallow apical crypts. Apical crypts in the salinity 20 and 30 groups had small apical membranes and undeveloped microridges, and were embolic obviously. Cytoplasm of mitochondrion-rich cells in the salinity 5 and 30 groups developed tubular systems and abundant cristae mitochondria. The tubular system of the salinity 20 group was non-spatially constant and had loose structure. Part of the tubular system contracted into a pearl bubble structure and shared rough endoplasmic reticulum. Mitochondrion-rich cells in Trachinotus ovatus under salinities 5 and 20 appeared both seawater-type’s and freshwater-type’s features, and those in salinity 30 had typical characteristics as seawater-type MR cells. Structural changes of mitochondrion-rich cells were suited to different osmotic pressure.