BACKGROUNGD: Alterations to mitochondria-rich cells (MRC) in the fish gill epithelium have been previously reported. OBJECTIVES: To specify the variation pattern of apical openings in mitochondria- rich cells in short and long terms exposure to different salinities. METHODS: Yellowfin seabream, (Acanthopagrus latus) was subjected to different salinities (freshwater, 5, 20 and 60ppt) besides the normal environmental salinity in the Musa creek (42ppt) over 21days, with three replicates for each condition. Samples were collected at the 1st, 7th and 21st days of experiment. Dissected gill arches were fixed in 2% glutaraldehyde+2% paraformaldehyde (pH=7.4) at 4°C and studied using scanning electron microscopy. RESULTS: Three subtypes of mitochondria-rich cells (shallow basin, deep hole and wavy convex) were detected in the gill epithelium based on different environmental salinities. While most of mitochondria-rich cells were present in seawater group, all of them were detected as the shallow basin subtype in 60 ppt group. Meanwhile, decrease in salinity to 20ppt, made morphological changes in the apical membrane of the mitochondria-rich cells on day 7, so that most of these cells have been detected as wavy convex or shallow basin subtypes. On the other hand, on day 21 they showed a pattern similar to the basal status. Furthermore,when they were transferred to hypoosmotic medium (5ppt and FW), rapid changes were exhibited in the apical membrane of mitochondria-rich cell which were stabilized after 21 days so that all subtypes of mitochondria-rich cells were observed in photomicrographs of gill filaments. CONCLUSIONS: Shallow basin mitochondria-rich cells as typical cells in seawater fish species, would be able to do osmoregulation in hyperosmotic environment in yellowfin seabream, yet in hypoosmotic conditions all sub- types of the mitochondria-rich cells would be required.