Gill histomorphological alterations were used to assess the effects of chronic exposure to HgCl2 on the yellowfin seabream, Acanthopagrus latus. In this regard, 90 A. latus were exposed to sublethal concentrations of HgCl2 (10, 20, 35 and 50 μg/L) for 3 weeks. Treated fish were erratic and showed respiratory distress. The most common morphological abnormalities included: filaments disorganization, increase of mucus secretion, debris and blood plaques on the filaments, losing or shortening of some filaments. The most frequent histopathological changes detected in the gills included extensive lifting of the lamellar epithelium and edema of lamellae with enlarged sub-epithelial spaces, exfoliated epithelium of lamellae, telangiectasia, hypertrophy and hyperplasia of the epithelial cell resulted in partial fusion of the secondary lamellae and a reduction of the water space, club shaping of gill lamellae, blood congestion. Some more severe alternations found in the gill of fish exposed to higher levels of HgCl2 (35 and 50 μg/L) included lamellar aneurysm and hemorrhages with rupture of the lamellar epithelium. According to the results of the present study, mercuric chloride could cause major histomorphological changes in the gill of A. latus, decreasing its gas exchange capability. Two mercury concentrations (10 and 20 μg/L) used in the present study were in agreement with the concentration of mercury in the water of different parts of Mahshahr creeks (the north of Persian Gulf) (3.66 to 15 μg/L). Therefore, based on the results the presence of pathological alteration in A. latus inhibited in the natural environment (Mahshahr creeks) seems to be logical.