Abstract: We observed the gill ultrastructure in early developmental stages of larval, juvenile and young grey mullets (Mugil cephalus) at 10th, 15th, 23rd, 36th, 40th and 44th day after hatching by scanning electron microscopy. The respiratory area was estimated, and the relationship was studied among body weight, body length and respiratory area. Results show that the gill ultrastructure and function gradually improved in larval, juvenile and young M.cepalus as growing. The total number of gill filaments increased with increasing body length and body weight in power series and logarithmic series, respectively. The ratio of secondary filaments per millimeter to body weight increased with increasing body weight in power series. Relationship between body weight and total respiratory area as well as that between body weight and single secondary filament area could be expressed by quadratic equation. In conclusion, development and differentiation of the gill were in accordance with growth, development and perfection of physiological function in larval, juvenile and young M. cepalus.