Abstract: The otolith microstructure of juvenile grass carp(Ctenopharyngodon idellus)sampled from hatchery and wild were examined and analyzed.The lapilli of juveniles could be divided into inner optically dense and outer low optically dense zones based on the difference of transparence.The size and increment numbers of optically dense zone in hatchery-reared individuals were bigger than that in wild individuals.Increments in optically dense and relatively low optically dense zones were both clear and regular in reared individuals.Increments were clear or unclear in optically dense zone,while clear,but with irregular width in low optically dense zone in wild individuals.The otolith opacity and transparence were probably controlled by average water temperature the juveniles experienced.The irregular width of increments in wild individuals was potentially induced by the fluctuations of food supply in nature.The difference of otolith microstructure could be used to discriminate between the reared and wild populations in the fish.