Based on the histological observations under optical microscope,we studied the ontogeny of the gill of rock bream Oplegnathus fasciatus at (24?1.0)℃.At hatching,gill anlage was visible in the pharyngeal region.On 2 DAH(Day After Hatching),as the branchial cavity was formed,four pairs of gill arches were evident and some muscular fibres were observed in gill arches.The blood cells were visible in the vascular structures of gills on 3 DAH,and the primordial filaments first appeared in the second and third arches,then in the first and fourth arches on 4 DAH.The primordial lamellae were first formed in the filaments of the second and third gill arches on 6 DAH,then in the first and fourth arches on 7 DAH.On 6 DAH,the pseudobranch anlage was observed as a paired structure lied by the anterior branchial cavity and the lamellae in the pseudobranch were visible on 7 DAH.The chloride cells were first arranged in the base of the lamellae on 7 DAH,then also in the base of filaments on 8 DAH.The pavement cells were visible in the branchial epithelium on 8 DAH and the pillar cells were developed to delimit the vascular structures of lamellae on 9 DAH.On 10 DAH,the mucous cells were seen in the lamellae of pseudobranch.From 14 DAH forward,the filaments and lamellae of gills increased in number and length prominently and the gill of larvae was similar to that of juvenile fish.The specific activity of Na⁺,K⁺-ATPase increased from cleavage period(0.712?0.400)U/g to segmentation period(2.315?0.515)U/g,and then decreased progressively to a low level until 9 DAH(1.389?0.734)U/g.Subsequently,the activity increased.After 12 DAH,it decreased to the minimum at (0.246?0.126)U/g on 14 DAH.Then the activity increased rapidly and reached the maximum at(4.731?0.309)U/g on 18 DAH.From 20 DAH,the specific activity of Na⁺,K⁺-ATPase reached a relatively stable level until 50 DAH(3.667?0.633)U/g.The change of Na⁺,K⁺-ATPase activity is closely related with the development of gill,especially the chloride cells.The mortality of rock bream larvae was very high in the period when the specific activity of Na⁺,K⁺-ATPase reached the minimum from 12 to 16 DAH.